US helicopter gunships circled low over the heart of Baghdad today, firing in support of American and Iraqi troops who claimed to have killed 50 suspected militants in a lengthy firefight.

Explosions and heavy machine gun fire could be heard across Baghdad after gunmen attacked Iraqi army checkpoints in a Sunni stronghold, and the Iraqis called in American back-up.

The Times saw Apache attack helicopters swooping and banking repeatedly over the lawless Haifa Street district on the western banks of the River Tigris, trails of smoke rising into the air from the ground below them.

The attack came ahead of President Bush’s expected announcement tomorrow of a ‘surge’ in US troop levels and as the Shia-led government prepares to launch the latest ‘Baghdad plan’ to crush insurgent violence in the troubled capital.

The violence coincided with the death of around 30 Turkish construction workers killed when their aircraft crashed trying to land in foggy weather north of Baghdad.

The chartered Moldovan Antonov-26 carrying 35 people, also carrying one American and five air crew, crashed around midday near the Sunni Triangle town of Balad after having aborted an earlier landing because of bad weather, Turkish Foreign Ministry officials said.

Balad is the main US military logistics centre in Iraq, around 50 miles north of Baghdad.

In the capital itself Major General Ibrahim Shaker, a Defence Ministry spokesman, said 21 militants were captured during the city centre battle, including seven foreign Arabs.

"Today's operation was designed to purge Haifa Street and nearby neighborhoods from terrorists," he said.

Ali al-Dabbagh, a spokesman for the Shia-led government, said Iraqi forces had decided to wipe out "terrorist hide-outs" in the Sunni area of Haifa Street - a once-affluent area of central Baghdad housing the elegant riverside former British Embassy once frequented by TE Lawrence and Gertrude Bell, and the Iraqi national museum looted in the days after the invasion.

"God willing, Haifa Street will never threaten the Iraqi people again," said Mr al-Dabbagh. He added that the Iraqi government had no objection to the expected increase of around 20,000 US troops.

Deploying American aerial and armoured power in support of Iraqi troops on the ground is the model likely to be used in the Iraqi government’s widely-trailed ‘Baghdad Plan’ to curb suicide bombers and insurgent attacks that have crippled political and economic progress in the country nearly four years after the US-led invasion.

"The goal is to protect Baghdad and other areas. If this is going to be achieved by an increase in friendly coalition forces, we have no objection and we support this," he said.

The US military said that its forces came under fire from rocket-propelled grenades and small arms during the Haifa Street operation, which lasted throughout the morning and afternoon.

Seeking to reassure Iraqis about the new ‘surge’ and President Bush’s recent replacement of his Defence Secretary, two top military commanders and the US Ambassador to Baghdad, a US military spokesman insisted: "There may be a lot of changes in leadership and there may be a lot of changes in tactics, but the relationship with our Iraqi counterparts is unchanging."

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