Turkish forces fought Kurdish guerrillas at close quarters as they advanced on a key PKK base in northern Iraq on Monday, and Baghdad warned a prolonged incursion could have serious consequences for the region.

Backed by warplanes, tanks, artillery and combat helicopters, troops killed 41 rebels on Monday, the Turkish General Staff said in a statement. This took the total PKK death toll since a major offensive began on Thursday to 153.

Close combat with the terrorists is continuing in two separate zones, it said. The troops in the critical zones of the operation were reinforced and some of the troops were replaced by fresh forces.

The Turkish military said 17 of its soldiers had died so far in the campaign, fought in harsh winter conditions.

Turkish troops are trying to root out Kurdistan Workers Party (PKK) guerrillas who have used the mountains of northern Iraq as a base for their fight for self-rule in the mainly Kurdish southeast of Turkey since the 1990s.

Iraq's National Security adviser Mowaffaq al-Rubaie said in Baghdad fighting could have very serious consequences for a part of Iraq that has been relatively stable compared with the rest of the oil-rich country.

The White House said Turkey's incursion, estimated to involve around 10,000 troops according to a senior Turkish military source and Turkish media, should be limited.

We hope that this is a short-term incursion so that they can help deal with the threat, White House spokeswoman Dana Perino told reporters in Washington.

Ankara launched a ground incursion on February 21 in a remote part of Iraq's largely autonomous region. It said Iraqi authorities had for years failed in undertakings to crack down on the rebels.

Pressure had mounted on the Turkish government after a series of deadly attacks on soldiers and civilians.

A Kurdish security official said Turkish troops and PKK rebels clashed during the night in the Amadiya area, 10 km (6 miles) south of the border.

TAKING PKK CAMPS

Turkish commandos were engaged in close battle 1.5 km (miles) from a key PKK command centre, used to store equipment and arms, in the Zap valley after heavy aerial bombardment, a senior Turkish military source told Reuters.

If it (Zap camp) falls, it will be a big blow to the PKK's morale, the senior source said, adding that 500-700 new troops were sent to Zap and Haftanin zones late on Sunday. He said troops would then move to the next target further towards the Qandil mountains, a PKK stronghold.

The advance was slowed by having to clear PKK land mines and remote control bombs, the Turkish military source said.

A PKK spokesman in northern Iraq, Ahmed Danees, said 81 Turkish soldiers and 4 rebels had been killed so far in the offensive. This could not be verified.

Turkey blames the PKK, defined as a terrorist group by the United States and the European Union, for the deaths of nearly 40,000 people since it began its armed struggle in 1984.

Turkey's deputy prime minister, Cemil Cicek, told reporters after a cabinet meeting that the troops would withdraw once they had completed their mission.

In Washington, Turkey's ambassador to the United States, Nabi Sensoy, also stressed the operation would be limited.

This is an operation limited in size, scope and duration, Sensoy told Reuters in a telephone interview, without being more precise over exactly how long it would last.

Thousands of Turks gathered in Ankara's main mosque on Monday to mourn the deaths of three army officers killed in the operation.

U.S. officials say Ankara has given assurances it will do all it can to avoid civilian casualties in northern Iraq. Washington has provided Turkey with intelligence to track down PKK rebels, estimated to number around 3,000 in northern Iraq.

There have been no reports of civilian casualties. But residents in villages near the border say they are being targeted in Turkish air strikes and artillery barrages.

So far the battle-hardened Iraqi Kurdish Peshmerga forces have stayed on the sidelines of the operation. Kurdish officials regard the remote mountainous area as outside their control.

While Iraqi Kurds have little sympathy for the PKK, there is widespread anger at the incursion. The leadership of Iraqi Kurdistan has said any targeting of Kurdish civilians would result in massive resistance by its Peshmerga forces.

Iraq has called for a diplomatic solution to the PKK presence, saying it has taken some measures to deal with the rebels but is focused on trying to stabilize the rest of Iraq.

Ankara says it has the right under international law to hunt and kill members of the PKK to avert attacks at home.

European Union President Slovenia called on Turkey to refrain from disproportionate military action and to respect Iraq's territorial integrity and human rights.

It said Turkey, which is trying to join the EU bloc, should limit its operations to fulfilling its main purpose of protecting the Turkish population from terrorism.

Iraq