A US military helicopter crashed in Baghdad tonight, although the US military said it did not appear to have been brought down by insurgents determined to wreck Sunday's elections.

As hundreds of thousands of Iraqi expatriates began voting in 36 cities around the world, nationalist and Islamic rebels kept up their campaign of violence, killing five US soldiers and at least 10 Iraqis in attacks around Iraq. Three US soldiers were killed by a roadside bomb as they patrolled western Baghdad.

But the Government said it had captured three insurgents with al-Qaeda links and a senior official Qassem Daoud, the National Security Adviser, said their next target was Abu Musab al-Zarqawi, the Jordanian terror leader who is Iraq's most wanted man. "His organisation is crumbling as security forces continue their efforts to kill or capture him," said Qassem Daoud, the National Security Adviser.

The US Army said an OH-58 Delta helicopter crashed in southwestern Baghdad at 7.55pm (4.55pm UK time). The fate of its two-man crew was not immediately known. The crash came just two days after 31 Marines died in another helicopter crash in western Iraq, the single deadliest incident of the war.

An estimated 1.2 million Iraqis living aboard are eligible to vote in the elections for a national assembly that will fix Iraq's democratic constitution, but less than a quarter have actually signed up. Of the 150,000 living in Britain, only a fifth have signed up for the first election since a US-led invasion force ousted Saddam Hussein last April.

At the London polling station at Wembley conference and exhibition centre, several dozen voters arrived within minutes of the doors opening at 7am. Cheers rang out after each voter cast their ballot.

Abdul Kraish, the first to vote, said the vote was an historic moment. "This is an event we have been waiting for all our lives, to participate in a free and fair election. We need to run our country in a civilised way."

Election officials in Iraq itself have no guarantees that voting can go off peacefully. One of the small group of international observers in Baghdad said this morning that the vote could well bring a "day of carnage".

Ashraf Kazi, the United Nations Secretary-General's special envoy to Iraq, admitted today that the security in Iraq meant there are areas where voter turnout could be low.

Yet he told BBC Radio 4's Today programme he was confident the Iraqi people would seize the historic opportunity to exercise their right to a free vote. "I hope it will be the first step on the road to a democratic Iraq," Mr Kazi said.

The UN would continue to play a role after the election, Mr Kazi said, as its mandate to promote dialogue between the country's political factions would continue.

Iraq has an estimated 14.1 million eligible voters. Ballot boxes have been distributed to 5,300-odd polling centres across the country, along with millions of ballot papers and tens of thousands of bottles of indelible ink, which voters will be tagged with to avoid repeat voting.

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