Thousands of Turkish army troops crossed the border with Iraq Friday in a major military operation against guerrilla forces of the Kurdish Workers Party (PKK), which has fought for decades against the Turkish state, seeking a separate state for the 15 million Kurds who inhabit southeastern Turkey.

The military action demonstrates the increasingly volatile conditions created by the US invasion and occupation of Iraq, which has destabilized the entire Middle East. The Turkish forces are operating within the battle space of the American army of occupation in Iraq, as well as on the territory of the Kurdistan Regional Government (KRG), whose militia, the pesh merga, is the most formidable local military force.

The attack was the first ground invasion of Iraq by Turkish troops since the United States invaded and occupied the country in 2003. Turkey conducted several major military sweeps against the PKK in the 1990s, while Saddam Hussein was still in power. Hussein backed the operations since he was himself engaged in bloody repression against Kurdish nationalist elements.

There were conflicting reports about the scale of the operation. The Turkish television network NTV, which is privately owned, said 10,000 troops were engaged in the offensive and had crossed six miles into Iraq in the Hakurk region, south of the town of Cukurca.

The Turkish press hailed the invasion in stridently chauvinist terms, with the daily newspaper Hurriyet reporting it under the headline, “Ten Thousand Heroes in Northern Iraq.”

The state-run Anatolia news agency reported the participation of Turkish bombers, helicopters and artillery as well as the ground troops, but said only 3,000 soldiers, the equivalent of two brigades, had actually entered Iraq. The rest were apparently deployed in support positions on the Turkish side of the border.

A PKK spokesman confirmed the invasion and reported Turkish casualties, both dead and wounded. An Iraqi border officer told the press that Turkish artillery had shelled Kurdish villages in the Sedafan area, some 20 miles from the border, with casualties still unknown.

The government of Turkish Prime Minister Recep Tayyip Erdogan called the operation “limited” and said it would be concluded as quickly as possible. Officials said that Turkey was committed to respecting Iraqi sovereignty, but had no choice to act because of cross-border raids by PKK guerrillas. More than 40,000 people have died in fighting between the Turkish army and the PKK, the bulk of them Kurdish civilians.

But it was not clear that Erdogan was in full control of the military command, which has repeatedly clashed with his Islamic-oriented Justice and Development Party (AKP).

A commentary in the Eurasia Daily Monitor noted, referring to the Turkish army command by its acronym, TGS, “It is currently unclear whether the decision to stage the operation was taken by the TGS or the Turkish civilian government. Speaking on Turkish television, several pundits suggested that the final decision to launch the operation must have been taken at the four-hour February 21 meeting of the National Security Council (NSC) ... However, the artillery bombardment began several hours before the NSC convened.”

It is thus quite possible that Erdogan’s hand was forced by the military commanders, who have been pressing him to authorize a ground assault on the PKK in addition to the sporadic air strikes conducted since last November, after a PKK raid that killed more than a dozen Turkish soldiers.

The timing of the ground invasion was surprising, given the extreme winter conditions, including heavy snow in the mountains and high winds, which negate much of the technical advantages of the Turkish army, a key component of NATO with extensive weaponry supplied by the United States.

The invasion produced different responses in the US and Europe, with the Bush administration and US military officials in Iraq largely supportive, while the European Union called on the Turkish government to exercise “restraint.”

White House officials said that Erdogan informed Bush of the ongoing military operation in a midnight phone call. The Turkish deputy chief of staff, Gen. Ergun Saygun, visited Washington this month and held talks with Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, in which the campaign against the PKK was undoubtedly discussed.

White House spokesman Scott Stanzel said Friday, “We were notified and we urged the Turkish government to limit their operations to precise targeting of the PKK—to limit the scope and duration of their operations—and we urged them to work, directly, with Iraqis, including Kurdish government officials, on how best to address the threat.”

The US military spokesman in Iraq, Rear Admiral Gregory Smith, said, “Multi-National Forces-Iraq is aware Turkish ground forces have entered into northern Iraq, for what we understand is an operation of limited duration to specifically target PKK terrorists in that region... The United States continues to support Turkey’s right to defend itself from the terrorist activities of the PKK and has encouraged Turkey to use all available means, to include diplomacy and close coordination with the Government of Iraq to ultimately resolve this issue.”

A spokeswoman for the European Commission was more cautious, saying, “Turkey should refrain from taking any disproportionate military action and respect human rights and the rule of law... The EU understands Turkey’s need to protect its population from terrorism. We encourage Turkey to continue to pursue dialogue with international partners.”

The British government urged Turkey to withdraw as quickly as possible while the German government said it viewed the incursion with “great concern.”

The most important response could come in Iraq itself, whose US-imposed government includes the two main Kurdish nationalist parties that share power in the Kurdish region which has now been invaded by Turkey.

Turkish President Abdullah Gul spoke with Iraq’s President Jalal Talabani, a Kurd, to reassure him that no long-term occupation of Kurdish territory was planned. Both the Iraqi regime in Baghdad, headed by Prime Minister Nouri al-Maliki, and the Kurdistan regional authority, headed by Massoud Barzani, urged Turkey not to carry out the military action. But Barzani ordered the Kurdish pesh merga not to fight the Turkish troops.

Despite these efforts, a clash between Turkish and Iraqi Kurdish forces nearly erupted Thursday, when hundreds of Turkish troops stationed inside northern Iraq since 1997 as monitors of a ceasefire between two rival Kurdish militias attempted to link up with the Turkish troops about to come across the border.

According to several press reports, the Turkish monitors were actively coordinating their actions with the Turkish high command, and one unit tried to bring its tanks and armored cars out of their base and take up positions to secure two major roads in the province of Dahuk, in Iraq’s northwest corner.

They were quickly confronted by a larger group of Kurdish pesh merga and a 90-minute standoff ensued, at which point the Turkish troops retreated and returned to their base, where they remain surrounded. A spokesman for the Kurdistan Regional Government said, “The government of Kurdistan ordered the pesh merga forces to be on alert in fear of any Turkish incursion on Iraqi territory.”

The day before the Turkish invasion, McClatchy Newspapers carried an account of a rare visit by a US journalist to a PKK-controlled area in the Qandeel Mountains, along the Iraq-Turkish border. The article said: “No Iraqi troops patrol here. PKK men in uniform check the IDs of those who seek to visit. The image of the PKK’s leader is emblazoned on a mountain slope, and a sign openly proclaims PKK headquarters. The pesh merga troops of the Kurdistan Regional Government, which officially rules northern Iraq , make no effort to enter.”

A PKK spokeswoman complained about US intelligence agencies supplying information to the Turkish military to assist the invasion, and appealed for “efforts to solve the Kurdish question through dialogue.”