Turkey (search) rejected on Sunday the demands of Islamic militants who are threatening to behead three of its kidnapped citizens during a visit by President George W. Bush (search) to Turkey. A bombing south of Baghdad killed more than 20 people.

With violence persisting in Iraq, security measures have been increased around government buildings, power stations and oil installations in advance of Wednesday's transfer of sovereignty to a new Iraqi administration, an official in the Interior Ministry, speaking on condition of anonymity, said. He refused to give details.

Followers of the most wanted Islamic militant in Iraq, Abu Musab al-Zarqawi (search), announced Saturday they had kidnapped three Turkish workers and threatened to behead them after 72 hours unless Turkish companies stop doing business with American forces in Iraq and called for protests in Turkey against Bush's visit.

Bush met with Turkish leaders in Ankara on Sunday ahead of a NATO summit starting the next day. Some 40,000 people protested in street demonstrations in Istanbul against Bush, whose policies in Iraq have been extremely unpopular among Turks.

Turkey's defense minister said Ankara would not negotiate with the hostagetakers.

"Turkey will not bow to pressure from terrorists," private CNN-Turk and TV8 television stations quoted Gonul as saying.

Secretary of State Colin Powell said Sunday that the United States is doing all it can to locate and free three Turkish nationals kidnapped by terrorists in Iraq.

"We hope it will be possible to rescue them, but it's a dangerous situation," Powell said in an interview from Turkey with CNN's "Late Edition."

Defense Secretary Donald H. Rumsfeld said the United States was not planning any immediate increase in troops to deal with the insurgency as a new Iraqi government takes power this week. He noted that the number of troops had already increased over the past three to four months from 113,000 to 141,000.

"We don't want to be an occupying power," he told the British Broadcasting Corp. "Breakfast with Frost" program from Istanbul. "The Iraqi people are going to have to provide for the security of their country and they are well on the way to doing it.

Iraq's interim prime minister, Iyad Allawi, said his government wanted to try to split the insurgency by dividing less ideological members from the hardcore leaders.

"We are drawing up plans to provide amnesty to Iraqis who supported the so-called resistance without committing crimes, while isolating the hardcore elements of terrorists and criminals and undercutting their base of support," Allawi wrote in an opinion article published Sunday in Britain's The Observer newspaper.

Allawi said Saturday that violence could force the delay of national elections, a key part of U.S. efforts to bring democracy to Iraq, scheduled to take place by Jan. 31 under Iraq's interim constitution. He also said his government was drawing up a law to give security forces more power to make arrests and impose curfews.

The U.S. military has posted a $10 million reward for help in capturing or killing al-Zarqawi, who is blamed for numerous bombings in Iraq and whose movement beheaded two previous hostages, an American and a South Korean.

The military said a pair of car bombs may have caused the explosion late Saturday in downtown Hillah, a largely Shiite Muslim city south of Baghdad. Iraqi police and Hillah-area hospitals reported 23 people were killed and 58 wounded. The U.S. military reported a death toll in line with the Iraqi count.

Elsewhere, three mortar shells exploded Sunday at the Mosul office of the Patriotic Union of Kurdistan, a pro-U.S. political party. Four party members were wounded. a PUK official said. A Mosul policeman was killed in a drive-by shooting in another incident, police said.

A strong explosion rocked central Baghdad on Sunday, and smoke rose from the U.S.-guarded Green Zone. The U.S. military said it had no details.

Explosions were also heard early Sunday on the northern outskirts of the troubled city of Fallujah, west of the capital. Residents said a U.S. Marine position was attacked by mortars and rocket-propelled grenades, but there was no confirmation from U.S. officials.

The bloodshed and the abductions threatened to cast a shadow over a NATO summit opening in Istanbul on Monday, where Bush is seeking the alliance's help in stabilizing Iraq.

The Arab television station Al-Jazeera aired a video issued by al-Zarqawi's "Tawhid and Jihad" organization, showing the three Turks kneeling on the ground in front of two black-clothed gunmen and a black banner emblazoned with the name of al-Zarqawi's organization. The men held up Turkish passports.

In a written statement, the group demanded Turkish companies stop doing business with American forces in Iraq and called for "large demonstrations" in Turkey against the visit of "Bush the criminal."

It said that if Turkey refused their demands the hostages "will receive the just punishment of being beheaded."

Al-Jazeera received the tape Saturday, an employee at the station told The Associated Press. The statement did not say when or where the three were abducted. It appeared the deadline was Tuesday, but the message did not specify what time it runs out.

The three men disappeared two days ago, said a Turkish consular official in Baghdad who asked to be identified only by his surname, Gungor. He said he had no further information.

And on Thursday, fighters loyal to al-Zarqawi launched a wave of coordinated attacks in five cities in Iraq, battling with U.S. troops who eventually regained control but only after some 100 people, including three Americans were killed.

Violence continued in one of the cities, Baqouba, where gunmen on Saturday attacked the offices of Allawi's political party and those of another, Shiite party. Three civilians and six insurgents were killed in fighting in the city.