Ankara: Turkish soldiers on Wednesday crossed into northern Iraq in pursuit of Kurdish rebels for the first time in two years and bombed targets across the border, killing four militants, the army said.

The cross-border operation came in response to an attack by the outlawed Kurdistan Workers' Party (PKK) on border guards near the Iraqi frontier, the latest in a series of bloody assaults by the rebels.

The attack on the base in the southeastern province of Sirnak at 1:00 a.m. (2200 GMT) triggered a clash lasting several hours in which a Turkish soldier was killed, the army said in a statement on its website.

Turkish commandos and special forces then followed the rebels after they fled into northern Iraq "after their unsuccessful attack", it added.

"Four terrorists were killed in clashes at the border and two to three kilometres" (1.2 to 1.8 miles) inside Iraqi territory, the army statement said.

The Turkish soldiers "are still continuing a sweep of the region," it added.

Turkish fighter jets also bombed a group of rebels as well as mortar and anti-aircraft gun positions detected deeper inside northern Iraq, successfully hitting the targets, the statement added.

The NTV news channel reported that 600 to 800 soldiers were taking part in the ground incursion.

The pro-Kurdish Firat news agency - considered by Turkey to be a PKK mouthpiece - said on its website that the rebels had confirmed a Turkish incursion of about two to three kilometres inside Iraq.

The agency also cited the rebels as saying that the Turkish army had suffered heavy losses in the attack on the border guards, but did not give a figure.

Ercan Citlioglu, an expert on terrorism and outlawed groups, said it appeared to be more of a short-term operation with the troops returning home after achieving their aims, rather than a large-scale lengthy incursion.

But he told the NTV news channel it was a "message" to Iraqi Kurds to take measures against the PKK, and also to the rebels that they would face consequences from attacking Turkish targets.

PKK rebels have long taken refuge in remote mountainous bases in Kurdish-run northern Iraq which they use as a launching pad for attacks against Turkish targets across the border.

The group, listed as a terrorist organisation by Ankara and much of the international community, has recently stepped up attacks against the security forces in Turkey's mainly Kurdish southeast.

In one of their bloodiest attacks in recent months, the rebels fired rockets on a navy base in the southern Mediterranean port of Iskenderun last month, killing six soldiers.

Turkey has often accused the Iraqi Kurds of tolerating and even aiding the PKK, but has recently replaced its hostile rhetoric with a policy of seeking cooperation with the local authorities to curb the rebels.

The head of the Kurdish administration of northern Iraq, Massud Barzani, pledged "all efforts" to stop PKK violence against Turkey when he paid a landmark visit to Ankara earlier this month after years of animosity.

The Turkish army has staged a series of air raids against PKK bases in northern Iraq since December 2007, often with the help of US intelligence, and in February 2008 carried out a week-long ground incursion.

Some 45,000 people have been killed in the conflict since 1984 when the PKK took up arms against Ankara for self-rule in Turkey's mainly Kurdish southeast.