The United States released some of a group of 11 Turkish special forces whose detention by U.S. troops in northern Iraq angered Turkey (search) and threatened to further strain already tense ties between the longtime allies.

U.S. officials remained silent over why the Turkish troops were seized in a Friday night raid by American forces on an office in northern Iraq. A Turkish newspaper said the detentions aimed to foil a Turkish plot to kill a senior official in the oil-rich city of Kirkuk (search).

Turkey closed a key border crossing into Iraq and its powerful military reportedly considered further steps to protest.

Prime Minister Recep Tayyip Erdogan (search) said Saturday evening that some of the soldiers had been released, but did not say how many.

"Some of the soldiers have been released. Some of them are still in their hands. Efforts (for their release) are continuing," Erdogan said during a visit to the northern Turkish city of Samsun.

The detentions came as Turkey is still trying to repair relations with the United States, at a low since the Turkish parliament's refusal in March to allow U.S. troops to use the country as a staging ground to invade Iraq.

The detentions also reflected the frictions between the two NATO (search) allies' differing interests in northern Iraq. U.S. forces have been working closely with Kurds in the area, while Turkey — facing a longtime separatist movement among its own Kurds — greatly fears an increase in Kurdish influence in Iraq.

Erdogan said earlier that Turkey wanted its troops released "as soon as possible."

"This is an ugly incident. It should not have happened," he said. "For an allied country to behave in such a way toward its ally cannot be explained."

In Washington, a State Department spokeswoman said U.S. Secretary of State Colin Powell discussed the detentions with Turkish Foreign Minister Abdullah Gul in a phone call.

Lt. Cmdr. Nicholas Balice, spokesman at the U.S. Central Command in Tampa, Fla., would not comment on the detentions, saying only, "We are certainly aware of incident, and at the moment we're investigating it."

Turkish government officials said about 100 American troops raided a Turkish special forces office in the town of Sulaymaniyah, detained 11 soldiers, and took them to Kirkuk.

The Hurriyet newspaper said the detentions followed reports that Turks were planning to kill a senior Iraqi official in Kirkuk. While there was no word on the identity, the city recently elected a Kurdish lawyer, Abdulrahman Mustafa, as its mayor amid concerns that the new administration may favor one ethnic group over another. The city is divided between Arabs, Kurds, ethnic Turks and Christians and has been the scene of ethnic tensions.

Turkey rejected any suggestion of a plot.

"These (reports) are nonsense. Such a thing is out of the question. Turkey is trying hard for stability in Iraq, not for instability. It is not possible for members of the Turkish Armed Forces ... to be involved in such a (plot)," Gul said.

After the arrests, Turkey closed down its border gate with Iraq at Habur, officials at the border said. The Habur crossing is used to ship U.N. aid as well as gas and other supplies to U.S. troops in northern Iraq. After the closure, trucks formed 6 mile-long queue at the border gate.

Private NTV television said Turkey's powerful military was discussing possible measures to take if the soldiers were not released — including closing Turkish airspace to U.S. military flights, stopping the use of the southern Incirlik air base and sending more troops into northern Iraq.

U.S. diplomat Robert Deutsch, who had been called to the Foreign Ministry to discuss the detentions, told reporters afterward that Turkish and U.S. officials were working for the soldiers' release, Anatolia reported.

Turkey has long maintained a military presence in parts of northern Iraq in a campaign to suppress Turkish Kurd rebels operating in the region.

At the onset of the U.S.-led invasion of Iraq, Turkey threatened to send in troops, fearing Iraqi Kurds would establish an independent state in northern Iraq, which could encourage Turkish Kurd separatists.

Kurdish rebels fought a 15-year war against Turkish troops for autonomy in Turkey's southeast, which has killed some 37,000 people. The rebels declared a unilateral cease-fire in 1999 after the capture of their leader, Abdullah Ocalan. The military rejected the cease-fire and sporadic fighting continues.

Turkey sent a team to Sulaymaniyah to meet with officials about the detentions.

"I hope there will be an outcome by this evening," Erdogan said.

It was the second time that U.S. forces detained Turkish soldiers in northern Iraq.

In April, the U.S. Army's 173rd Airborne Brigade caught a dozen Turkish soldiers, dressed in civilian clothes and trailing an aid convoy. U.S. forces suspected that the Turkish team was sent in to inflame local ethnic Turks, who already have tense relations with the city's Kurds and Arabs.

"Turkish-American relations date back to many years. Such tasteless incidents between two allies would not benefit anyone," Gul said.