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DIYARBAKIR, Turkey, July 13 (Reuters) - Turkey's army has boosted troop levels in the southeast to more than 200,000, most of them stationed along the border with Iraq, security sources told Reuters on Friday.

The unusually large-scale buildup, which includes tanks, heavy artillery and aircraft, is part of a security crackdown on Kurdish rebels hiding in southeast Turkey and northern Iraq, said the security sources, who declined to be named.

NATO member Turkey has refused to rule out a possible cross-border operation to crush Kurdistan Workers Party (PKK) rebels, despite opposition from the United States and Iraq.

The military General Staff in Ankara was not immediately available for comment on the troop numbers.

The armed forces usually boost troop levels in the spring when PKK rebels based in northern Iraq cross the mountains to carry out attacks against Turkish targets.

But security forces said the current buildup was larger than normal. One source said that in Sirnak province troop levels were as high as 50,000 compared with 10,000-20,000 normally.

Highlighting Turkey's security dilemma, army sources said on Friday two soldiers and two separatist rebels from the PKK had been killed in eastern Turkey during military operations over the past two days.

Dozens of soldiers and paramilitary police have been killed in an escalation of violence in east and southeast Turkey in recent months.

The army has repeatedly urged the government -- which faces elections this month -- to launch an operation against an estimated 4,000 PKK rebels based in northern Iraq.

Amid public anger over the deaths, Prime Minister Tayyip Erdogan has hinted parliament could be recalled to approve an operation but with just over a week before a parliamentary election has made no move yet to do so.

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