The US general who masterminded a troop surge in Iraq said on Wednesday that Afghanistan was "no more hopeless" than Iraq was before the 2007 campaign.

General David Petraeus, head of US Central Command, drew the comparison days after President Barack Obama ordered 30,000 additional troops to Afghanistan in a move partly modeled on the surge in Iraq.

Petraeus said violence was at a higher level in Iraq on the eve of the troop buildup there two years ago than in Afghanistan, and that Taliban insurgents commanded less popular support than their Iraqi counterparts.

"While certainly different and, in some ways tougher than Iraq, Afghanistan is no more hopeless than Iraq was when I took command there in February 2007," the general said.

"Indeed, the level of violence and number of violent civilian deaths in Iraq were vastly higher than we have seen in Afghanistan," he told the Senate Foreign Relations Committee.

The general warned that violence would rise initially as troops moved against Afghan insurgents and that progress would be "slower in developing than was the progress achieved in Iraq."

"Nonetheless, as with Iraq, in Afghanistan, hard is not hopeless," he said.

Petraeus endorsed Obama's plan to send reinforcements to Afghanistan, saying he believed it would break the momentum of Islamist insurgents allied with Al-Qaeda.

But he appealed for patience, saying it would not be possible to judge if the Afghan strategy was successful until December 2010.

In the short-term, stepped-up combat operations and anti-corruption efforts would trigger more unrest and political turmoil, he said.

"As in Iraq, the situation is likely to get harder before it gets easier," he said.

The general rose to prominence during the Iraq war, defending the risky surge strategy at tense congressional hearings, and his words tend to carry weight with lawmakers.

The US military has credited the Iraq surge two years ago with improving security and helping to prepare the way for a drawdown in American forces there.

Obama's deputies cite the Iraq troop surge as a model for the NATO-led mission in Afghanistan, where foreign troops are due to swell to 150,000 after more American and allied forces deploy in the next several months.

Skeptics of Obama's move argue Afghanistan presents far tougher conditions than in Iraq, with grinding poverty and a Taliban insurgency that has deep roots in Afghan society as well as mountainous terrain and harsh winters.

Petraeus, who oversees Afghanistan and the Middle East as chief of Central Command, said the mostly Pashtun Taliban insurgency enjoyed less public support among fellow Pashtuns than Sunni or Shiite militants did in Iraq during the peak of unrest there.

And the Taliban had "virtually no support among Afghanistan's other ethnic groups," he said.

The commander of US and NATO forces in Afghanistan, General Stanley McChrystal, meanwhile said his strategy only required the Kabul government to hold key roads and towns and not "every square inch" of territory.

"What they have to do is control enough of the population, enough of the key production and lines of communications, and establish enough credibility and legitimacy so that the insurgency can't be an existential threat," McChrystal told National Public Radio.

Over time, he said, "the insurgency loses relevance."

At the Senate hearing, Petraeus said shoring up the Afghan government's credibility posed a challenge amid widespread corruption and "serious abuse of power by some individual leaders."

President Hamid Karzai's re-election after a flawed poll in August had "further undermined confidence in the government," he said.

Petraeus also said recruiting and training a larger Afghan army and police force would be crucial to an eventual US drawdown but that it was too early to set a firm target figure for the size of an expanded force.

US commanders had previously called for expanding the army and police to a combined force of about 400,000 but Petraeus said that figure represented an aspiration.

Recruiting and retention rates would have to be assessed over coming months to ensure goals for expanding security forces were realistic, he said.