Militants loyal to suspected al Qaida terrorist Abu Musab al-Zarqawi have threatened to behead three Turkish hostages, fanning tensions as US President George Bush arrived in Turkey.

The Arab television station Al-Jazeera broadcast a video issued by the kidnappers, showing the three Turks in front of two black-clothed gunmen and a black banner emblazoned with the name of al-Zarqawi's organisation. The men held up Turkish passports.

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

The abductions have stoked anti-war sentiment in Turkey, where President Bush is already extremely unpopular.

More than 40,000 demonstrators, some chanting "US get out of the Middle East" gathered in an Istanbul square yesterday to protest against Mr Bush's visit to the country for a Nato summit, which opens today.

Hours ahead of his arrival in Ankara, police battled against protestors, eventually firing tear gas to disperse them.

In another hostage-taking incident last night, insurgents threatened to behead a Pakistani if US forces did not release some prisoners being held in Iraq within the next three days.

Three masked gunmen, in a declaration broadcast by the Arab language television station Al-Arabiya, said the countdown began with the screening of the tape.

Meanwhile, in Baghdad, the death toll rose to more than 20 after an explosion. The military said two car bombs may have caused the explosion late on Saturday, in Hillah, a largely Shi'ite Muslim area. Iraqi police and Hillah hospitals reported 23 people were killed and 58 wounded.

Elsewhere, three mortar shells exploded yesterday at the Mosul office of the Patriotic Union of Kurdistan, a pro-US political party. Four party members were wounded.

A Mosul policeman was killed in a drive-by shooting in another incident, police said.

A strong explosion later rocked central Baghdad, and smoke rose from the US-guarded Green Zone.

The bloodshed and abductions threatened to cast a shadow over the summit in Istanbul, where President Bush is seeking the alliance's help in stabilising Iraq.

The US has blamed much of the violence on al-Zarqawi's Tawhid and Jihad movement, and reiterated appeals to the Iraqi people to come forward with any information that might lead to his capture.

Iraq's interim prime minister, Ayad Allawi, warned that if security did not improve, it may become necessary to delay national elections set for January - a landmark in the path to democracy before the handing over of power to the Iraqis on Wednesday.