BAGHDAD — Turkey sent hundreds of troops across the border into the frigid mountains of northern Iraq yesterday, saying it inflicted heavy losses on Turkish Kurd rebels in the small-scale incursion and from air strikes two days earlier.

The offensive puts more pressure on Washington to mediate between Iraq and Turkey, two key allies in an unstable region. In a sign of increasing tension, the U.N. High Commissioner for Refugees reported that more than 1,800 people fled their homes in parts of Iraq’s semi-autonomous Kurdistan last weekend.

International oil prices fell sharply late yesterday after reports from Kurdish officials that the Turkish ground troops had withdrawn, easing fears that the conflict would disrupt oil supplies from the region. The Turkish military, however, did not confirm a pullout.

Yesterday’s raid was the first confirmed Turkish ground operation targeting rebel bases inside Iraq since the American invasion that toppled Saddam Hussein, though about 1,200 Turkish military monitors have operated in northern Iraq since 1996 with permission from local authorities.

However, the incursion did not represent a large-scale push that some feared could destabilize a relatively calm part of Iraq — and which is adjacent to the nation’s main northern oil fields around Kirkuk.

In November, the Turkish military reportedly massed 100,000 troops along the border, and there are fears that a major Turkish offensive could cause civilian casualties and lead to conflict with the Peshmerga, the fierce Kurdish regional militia.

The rebels, known as the Kurdistan People’s Paraty, or PKK, have battled for autonomy for southeastern Turkey for more than two decades and use strongholds in northern Iraq for cross-border strikes. Turkey has said it can no longer tolerate the attacks on its troops, and in October Turkey’s Parliament authorized the country’s military to strike back at the rebels inside Iraq.

The latest operation involving about 300 soldiers began about 3 a.m. and lasted 15 hours before the soldiers returned to Turkey, Iraqi Kurd officials said.

“Today’s Turkish military operation was a limited one, and the troops withdrew from Iraqi territory,” a spokesman for Kurdistan’s Peshmerga militia, Jabar Yawar, said.

In its statement, the Turkish military said ground forces based close to the border crossed “a few kilometers” into northern Iraq after spotting a group of rebels trying to infiltrate into Turkey overnight.

“A heavy blow was inflicted on the group with the land forces stationed in the area,” it said.

The military said it was not able to give the number of rebels who may have been killed during Sunday’s airstrikes on PKK targets, but maintained that “many facilities harboring the PKK were hit.”

“The PKK has suffered heavy losses in terms of its infrastructure and its human resources,” it said, insisting that no civilian targets were harmed.

It was unclear what role the U.S. played, if any, in yesterday’s ground operation.

American military commanders in Iraq didn’t know Turkey was sending warplanes to bomb in northern Iraq on Sunday until the planes had already crossed the border, said defense and diplomatic officials, who were angered about being left in the dark.

Americans have been providing Turkey with intelligence to go after Kurdish rebels in northern Iraq. And a “coordination center” has been set up in Ankara so Turks, Iraqis and Americans can share information, two officials said yesterday.