The icasualties.org website, which tracks military deaths in Afghanistan and Iraq, said 54 US soldiers had died in the war so far this year, as the coalition tries to regain momentum in its eight-year campaign. It was the worst toll for two consecutive months since the US-led invasion of 2001 and compared with a toll of 30 in the same period last year.
The Pentagon announced the latest American service member to be killed was Cpl Gregory Stultz, 22, who died on Feb 19 from small arms fire in Helmand province.
The top-ranking US military officer, Admiral Mike Mullen, warned of more casualties as 15,000 US-led forces continued Operation Moshtarak in Marjah, a key Taliban stronghold where foreign troops have faced strong militant resistance.
He also said efforts against the Taliban were "messy" and "incredibly wasteful", as was war in general, adding: "But that doesn't mean it's not worth the cost.
"We must steel ourselves, no matter how successful we are on any given day, for harder days yet to come."
In an operational update, Nato said: "Afghan and combined forces continue to encounter small but determined pockets of resistance, often from bunkers or other fortified positions."
Last year was the bloodiest yet for international troops, with 519 killed, up more than 75 per cent on the previous year.
Britain has the second highest toll of deaths, with 264, while Canada has the highest in proportion to the size of its force.
Gen David Petraeus, head of the US Central Command region that overseas the war, also tried to brace Americans for more casualties because of the extra 30,000 forces sent by President Barack Obama to pursue an aggressive counter-insurgency strategy.
"These types of efforts are hard, and they're hard all the time," he said. "I don't use words like 'optimist' or 'pessimist'. I use 'realist'."
The passing of a grim milestone in the war will be a reminder to the White House that the much of the public and the Left-wing of Mr Obama's Democratic Party have a limited tolerance for a war which has dragged on for years without tangible progress since the very early success of removing the Taliban government in retaliation for supporting Osama bin Laden and al-Qaeda. The casualty rate has not however yet reached the point it did in Iraq that drove public opinion resolutely against the war.
The US embassy in Kabul declined to comment on the growing death toll.
The Pentagon's tally tends to lag behind the well-regarded icasualties.org website as it does not officially record a death until 24 hours after next of kin are notified.
Fighting in Marjah continued on Tuesday as Marines managed to link up from the north and south after negotiating their way through mined poppy fields for more than a week. However, insurgent units appeared to have regrouped in the north. A bomb strapped to a bicycle in Lashkar Gah, the capital of Helmand, killed eight people.
Gen Stanley McChrystal, the Nato commander, went on television to apologise for a mistaken air strike on Sunday that killed 27 Afghan civilians including women and children.
The apology was an effort to quell rising public anger over civilian casualties after the third mistaken bombing raid in a week.
His one-minute video was dubbed into the local Dari and Pashto languages.
