A statement, posted on the website of the Ansar al-Sunna group and co-signed by the Islamic Army in Iraq and the Army of the Mujahideen, told Iraqis not to take part in "the farce of democracy and elections". It said: "Anyone who accepts to take part in this dirty farce will not be safe."

The statement described democracy as "un-Islamic" and polling stations as "centres of atheism".

With elections due on January 30, the threats will do little to encourage the Sunni Arab minority to vote. Their participation is seen as essential to the elections' credibility.

Iraq's biggest Sunni party, the Iraqi Islamic party, withdrew from the elections last week saying violence in central and northern Iraq made a free and fair vote impossible.

The Shia and Kurdish communities are likely to turn out in much greater numbers.

Up to 14 million Iraqis are expected to go to the polls to elect a 275-seat transitional assembly, whose chief task will be to draft a permanent constitution. On the same day, there will also be provincial governorate elections while Kurds, in the north, will vote to elect a Kurdistan assembly.

Determined to derail the electoral process, insurgents have killed election workers as well as candidates.

"This [vote] is a mockery by the enemy to grant legitimacy to the new government which serves the crusaders," the joint statement said.

"Participating in these elections... would be the biggest gift for America, which is the enemy of Islam and the tyrant of the age."

The groups that signed the proclamation have been among the most active insurgents in Iraq since the US-led invasion. Between them, they have claimed responsibility for scores of suicide bombings, kidnappings, assassinations and attacks on security forces.

Ansar al-Sunna is thought to have been responsible for last week's suicide attack on a mess tent at a US base in Mosul. The Islamic Army in Iraq kidnapped the two French journalists who were released last week.

A senior interior ministry official in Baghdad said: "Until now [these groups] are thought to have operated independently of each other, but this message may indicate they are now cooperating together, and with al-Qaida, tactically and strategically around the election issue."

On Monday, the Arab TV station al-Jazeera aired an audio tape purportedly by Osama bin Laden, in which he also called for a boycott of the vote and endorsed Abu Musab al-Zarqawi as his lieutenant in Iraq.

One senior Iraqi minister appeared sanguine, saying: "The battle lines are being clearly drawn.

"Are you with democracy and a say in your future, or are you with slavery to al-Qaida and a miserable death?"

As polling day nears, the interim government and American forces are desperate to demonstrate that they are making headway against the insurgents.

Yesterday they announced the capture of "a senior member" of Zarqawi's group in a raid in Baghdad.

A government statement said Fadil Hussain Ahmed al-Kurdi, 26, had been captured with two other insurgents. It said Mr Kurdi, also known as Ridha, was the brother of Umar Baziyani, a militant arrested in May.

"Ridha was responsible for facilitating communications between al-Qaida and the Zarqawi terror networks as well as coordinating the movement of terrorists in and out of Iraq," the government said.

The claim that officials from Saddam's regime and jihadis are working together received a boost yesterday when Christian Chesnot, one of the French journalists released last week, told the Arabic newspaper Asharq al-Awsat that his captors had been "fundamentalists" and former Ba'ath party loyalists.

"The group, as far as we understood, was made up of former Ba'athists, including the bodyguard of the personal secretary of Saddam Hussein," Chesnot was quoted as saying.

"In addition, there are also youths who told us they had been trained in Afghanistan in making explosives and who re ferred to Osama bin Laden as Sheikh Osama."

Iraq's former ambassador to India, Salah al Mukhtar, who is thought to be in hiding in Syria, backed the insurgency this week.

He told Hawlati, an independent Kurdish weekly, in an interview: "Not only has the Ba'ath party been successful in reorganising its ranks, but it [is] capable of withstanding occupation strikes and striking back more powerfully."

Asked whether the Ba'ath party supported Zarqawi, Mr Mukhtar said: "We support anyone who takes up arms against the US colonial rule of Iraq."