By Daren Butler

CIZRE, Turkey, June 7 (Reuters) - The rocky hills of Sirnak province echoed with the boom of tank shells fired by Turkish troops on Thursday in military exercises near the Iraqi border.

Reports of a cross-border operation against Kurdish rebels hiding in northern Iraq increased tensions in the area on Wednesday although Turkey denied it had launched a major incursion.

The army announced late on Wednesday a ban on all land and air travel between Iraq and three Turkish provinces, including Sirnak, as part of its operations against rebels of the banned Kurdistan Workers Party (PKK).

A Turkish newspaper said the "temporary security area", which will be locked down between June 9 and September 9, would also be used for air force training exercises.

In a small-scale operation near the town of Cizre on Thursday, the military assembled five tanks and about a dozen other military vehicles on the banks of the Tigris. Cizre is some 50 km (30 miles) from the Iraqi border gate of Habur.

Tank commanders fired across a deserted, arid valley towards targets placed on a distant slope.

Nearly 20 soldiers took shelter from the baking sun under the canopy of a large tent nearby to watch the exercises, which local sources said were into their third day.

A Turkish military source said on Wednesday troops had conducted a "limited" cross-border operation against PKK rebels hiding in northern Iraq. Neither the government nor the army General Staff confirmed the operation and officials in Ankara said there were no plans for a major incursion at present.

The exercise, scheduled to finish on Friday, is part of regular monthly drills.

The General Staff has described the recent build-up of forces near the border as routine. Every spring it sends more troops and equipment to the area to combat PKK fighters who cross into Turkey as the mountain snows melt, it said.

The head of the General Staff had called on the government in April to authorise an incursion to crush the rebels.

The increased border activity has rattled investors, who fear a major incursion would wreck Ankara's relations with the United States and the European Union, and destabilise mainly Kurdish northern Iraq, a relatively calm area of the country.

Prime Minister Tayyip Erdogan stressed parliament must authorise any Turkish military action outside the country's borders. Parliament is now in recess ahead of a national election on July 22.

But the government is under public pressure to crack down harder on PKK rebels, who this week killed seven paramilitary policemen in an attack on their headquarters.

Ankara blames the PKK for more than 30,000 deaths since the rebels launched their armed campaign for an ethnic homeland in southeast Turkey in 1984. The United States and EU also classify the PKK as a terrorist organisation.

AlertNet news is provided by