IRANIAN forces were involved in the recent battle for Basra, General David Petraeus, the US commander in Iraq, is expected to tell Congress this week.

Military and intelligence sources believe Iranians were operating at a tactical command level with the Shi’ite militias fighting Iraqi security forces; some were directing operations on the ground, they think.

Petraeus intends to use the evidence of Iranian involvement to argue against any reductions in US forces.

Dr Daniel Goure, a defence analyst at the Lexington Institute in Virginia, said: “There is no question that Petraeus will be tough on Iran. It is one thing to withdraw troops when there is purely sectarian fighting but it is another thing if it leaves the Iranians to move in.”

US defence chiefs are concerned that the troop surge has overstretched the military. Admiral Mike McMullen, chairman of the joint chiefs of staff, warned that the army and marines were at risk of crossing an “invisible red line” if the burden on forces remained. He said deployments of 15 months had to be reduced to a year “as fast as possible”.

Petraeus is likely to announce that combat tours will be reduced from 15 months to 12 months.

The number of US troops in Iraq is set to fall from 160,000 to 140,000 by July, but Petraeus is expected to recommend an indefinite pause in further troop cuts.

Moqtada al-Sadr, the radical Shi’ite cleric, has called for 1m people to march on Baghdad on Wednesday – the fifth anniversary of the fall of the capital – when Petraeus and Ryan Crocker, the US ambassador to Iraq, will be briefing Congress.

A senior Iraqi official who met Petraeus last week said, “It will be difficult to show that the situation is improving.” Another Iraqi source described the US general as “furious” that al-Maliki moved against the militias into Basra without consultation and had to rely on US forces to bail him out.

Abu Ahmed, a senior military commander with the Awakening, the Sunni tribal movement cooperating with US forces, said progress was largely the result of al-Sadr’s Mahdi Army ceasefire.

“When the Mahdi Army decides to resume its activities, neither the American troops nor the Iraqi government will be able to stop it,” he said.

Additional reporting: Hala Jaber

Times Recommends

• 'Lost' cargo ship Arctic Sea gives up its secrets
• Army wants to retreat in Afghanistan
• Jungle angel was Obama’s mother