CIZRE, Turkey — Supported by air power, Turkish troops crossed into northern Iraq on Friday in their first major ground incursion against Kurdish rebel bases in nearly a decade. But Turkey sought to avoid confrontation with U.S.-backed Iraq, saying the guerrillas were its only target.

The offensive, which started late Thursday after aircraft and artillery blasted suspected rebel targets, marked a dramatic escalation in Turkey's fight with the PKK rebel group.

A military officer of the U.S.-led coalition in Iraq said on condition of anonymity that several hundred Turkish soldiers had crossed the border. The coalition has satellites, drones and other surveillance aircraft at its disposal.

It was not possible to independently confirm the size or scope of the attack on the Kurdistan Workers' Party, or PKK, which is considered a terrorist group by the United States and European Union. CNN-Turk television, citing Turkish security officials, said the operation could last two weeks.

Late in the day, the Turkish military said five of its soldiers and 24 rebels had died in a clash inside Iraq and estimated at least 20 more rebels were killed by artillery and helicopter gunships. It said sporadic fighting was continuing.

The advance was the first confirmed Turkish military ground operation in Iraq since the U.S.-led invasion that toppled Saddam Hussein in 2003.