An American official in Washington, who agreed to discuss the tense situation along the frontier only if not identified, said the reports of a border crossing should be treated with skepticism.
The American said some Turkish officials might be feeling pressure to show increasingly angry Turks that the government is responding to an escalation of attacks by PKK rebels, who are fighting for autonomy in Turkey's heavily Kurdish southeast. On Monday, for example, Kurdish rebels struck a Turkish outpost and killed seven soldiers.
Three Turkish officials described the cross-border operation as a "hot pursuit" raid limited in scope, and one of them said troops had returned to their bases by the end of the day.
The officials spoke on the condition of anonymity because they were not authorized to speak to journalists.
Turkish authorities rarely acknowledge such military operations against the PKK, but the army has conducted brief raids across the border in the past.
The reports were likely to heighten anxieties over whether Turkey is planning a large-scale invasion. The last such operation was in 1997 and involved 50,000 soldiers.
Turkish leaders have said they are considering an offensive and have sent more troops and equipment to the frontier. But they hope the United States and Iraqi Kurds will stage their own crackdowns on the separatists, who raid southeastern Turkey after resting, training, and resupplying in Iraq.
Washington lists the PKK as a terror group; however, most US troops in Iraq are busy dealing with violence elsewhere, and most of the 16,500 US soldiers in the north are training Iraqi forces.
The Iraqi Kurd administration, meanwhile, has testy relations with Turkey, which has accused it of backing the PKK movement.
One Turkish security official said 600 commandos entered Iraq before dawn after Kurdish rebels shot at Turkish patrols near the Turkish border town of Cukurca.
Two other officials said troops went less than 2 miles into a remote, mountainous area in Iraq.
An Iraqi Kurd security official in the Shanzinan area said 150 Turkish soldiers occupied a mountain about a half mile inside Iraq for an hour and then left. The official, speaking on the condition of anonymity, said 200 other Turks staged a similar cross-border operation around the same time in the nearby Sirzeri area.
It was not immediately clear whether the Kurdish official referred to the same raid cited by the three Turkish officials.
Turkey's private NTV television quoted Foreign Minister Abdullah Gül as saying the reports of a cross-border operation were false.
"There is no such thing, no entry to another country. . . . We are in a war with terror. We will do whatever is necessary to fight terrorism," Gül said.
Officials at the Pentagon said they had seen nothing to confirm an operation by Turkish troops.
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