Barack Obama praised the contribution of British troops in Afghanistan today but warned there was "still a long way to go".

The US president's comments came after 15 British soldiers were killed in the first 11 days of July – including eight in 24 hours on Friday – taking the UK military death toll in the country since October 2001 to 184, five more than the lives lost by British forces in Iraq.

The deaths have increased concerns over the strategy in Afghanistan, in particular whether there are sufficient troop numbers and the forces are properly equipped.

"The contribution of the British is critical, this is not an American mission," Obama told Adam Boulton on Sunday Live on Sky News. "The mission in Afghanistan is one that the Europeans have as much, if not more, of a stake in than we do. Certainly the Afghans as well as the Pakistanis have more of a stake than we do. The likelihood of a terrorist attack in London is at least as high, if not higher, than it is in the United States. And that's the reason why Tony Blair and Gordon Brown and others have made this commitment."

The US president said his "heart obviously goes out to the families of those British soldiers". "We knew that this summer was going to be tough fighting, that there was an interest in the Taliban exerting control, they have I think been pushed back, but we still have a long way to go," Obama said. He said there would be an evaluation of tactics after the Afghan election.

The Ministry of Defence is considering a troops surge and a review of tactics.

In an interview with the British Forces Broadcasting Service today, the prime minister, Gordon Brown, echoed Obama's comments that the campaign in Afghanistan was necessary to protect Britain from the threat of terrorism. He insisted that the Operation Panther's Claw offensive to drive the Taliban from central Helmand province was making considerable progress.

"I think the operation we are engaged with is showing signs of success. Our troops are making progress as they attempt to make the area safe," he said.

"The reports that I have show that despite the loss of life – and it is tragic and it is very, very sad indeed – our forces are doing a magnificent job in moving forward."

Last week, the Liberal Democrat leader Nick Clegg said young lives were being "thrown away" in Afghanistan.

Today, Brown acknowledged there was public concern about the campaign, but he insisted it was part of a "clear strategy" to remove the terrorist networks from Afghanistan and Pakistan in order to protect the streets of Britain.

"It comes back to terrorism on the streets of Britain. If we were to allow the Taliban to be back in power in Afghanistan and al-Qaida then to have the freedom of manoeuvre it had before 2001, then we would be less safe as a country.

"There is a line of terror – what you might call a chain of terror – that links what's happening in Afghanistan and Pakistan to the streets of Britain."

Clegg also said that British troops in Afghanistan were "being bailed" out by newly arrived US forces - a claim that the defence secretary, Bob Ainsworth, angrily rejected today.

"This is not an American takeover, I think that's quite disgusting for people to suggest," Ainsworth told BBC News. "The notion that the Americans came to bail us out in Helmand province is an extraordinary thing for as politician to say."

He insisted that Britain had "just about the most capable armed forces in Afghanistan".

"They will tell you that they have seen a massive improvement in personal kit and equipment...and vehicles," he said.