Image caption General Petraeus (l) is tipped to replace Leon Panetta (r) at the CIA

Gen David Petraeus, the US head of international forces in Afghanistan, will be nominated as CIA director when its chief moves to head the Pentagon, unnamed US officials have said.

CIA director Leon Panetta will be nominated to take over as US defence secretary when Robert Gates retires in 2011, the sources said.

The changes will be officially announced on Thursday.

Mr Gates was first appointed as defence secretary by George W Bush in 2006.

The shuffling of the top national security jobs has been the subject of weeks of speculation.

Analysis It has been rumoured for some time that Gen Petraeus would be on the move by the end of the year. The question was whether he would become the next chairman of the US joint chiefs of staff, or head somewhere else. The rumour has been that Mr Obama favours the current vice-chief at the Pentagon, Gen James Cartwright, to take over the top military job, so director of the CIA looked a more likely slot for General Petraeus. Exact timing will be key. There are still important issues in Afghanistan over the summer to do with the campaign, the strategy, and troop withdrawals. But there will be a vacancy at the CIA, it seems. Leon Panetta's challenge will be greater, and his reported nomination is more significant. He has a hard act to follow in Robert Gates, who is widely respected as both a deft and safe pair of hands. The Pentagon also faces huge challenges, not least to its budget.

President Barack Obama will also nominate veteran diplomat Ryan Crocker as the next US ambassador to Afghanistan, NBC News reported.

The positions would require confirmation in the US Senate.

'I'll salute smartly'

Gen Petraeus' office in Kabul told the BBC it had "no comment at this time" on the reports out of the US.

In a BBC interview earlier this month, the general said he had "committed to staying here through the fighting season", which ends in late November.

"At this point in life you serve at the pleasure of the elected leaders above you and when they decide what the future holds obviously I'll salute smartly and execute their decisions," Gen Petraeus said.

"I've had discussions with individuals but it wouldn't be appropriate to comment."

US networks and AP reported that Gen Petraeus would be replaced in Afghanistan by Lieutenant General John Allen, currently deputy head of US Central Command - the command unit covering central Asia and the Middle East.

The changes would not take place immediately, but are expected to occur over the summer.

Lt Gen Allen is already in Washington DC, and Gen Petraeus is reportedly headed to the US capital from Afghanistan.

The announcement comes less than a year after Gen Petraeus took over leading Nato forces in Afghanistan from Gen Stanley McChrystal.

Meanwhile, Afghanistan's spring fighting season is ramping up, testing Nato and Afghan national army territorial gains.

In July, the US is expected to begin what US President Barack Obama has called a "significant" withdrawal of troops from Afghanistan, turning over security duties to Afghan military forces.