British pilots in Afghanistan are firing an increasing number of "enhanced blast" thermobaric weapons, designed to kill everyone in buildings they strike, the Ministry of Defence has revealed.

Since the start of this year more than 20 of the US-designed missiles, which have what is officially described as a "blast fragmentation warhead", have been fired by pilots of British Apache attack helicopters. A total of 20 were also fired last year after they were bought by the MoD from the Americans last May.

The missiles are a variant of the AGM-114N Hellfire missile, described by the Pentagon as "designed to produce higher sustained blast pressure in multi-room structures.

It adds: "The enhanced blast from the … warhead is more effective against non-traditional targets; multi-room structures expected in military operations in urban terrain operations, caves, and fortified bunkers."

The missile's warhead is made with a mixture of chemicals rather than a simple blast mechanism.

"The thermobaric Hellfire missile can take out the first floor of a building without damaging the floors above, and is capable of reaching around corners," according to Global-Security.org, a US thinktank.

It describes the effects of the missile as "formidable". Unlike conventional warheads, it produces a sustained pressure wave. US forces have deployed the missiles in Iraq as well as Afghanistan.

Its wider use was disclosed by John Hutton, the defence secretary, in answer to a parliamentary answer from Nick Harvey, the Liberal Democrat defence spokesman. "Given the MoD's reluctance to admit they were even going to use these weapons, they now seem to be getting rather more trigger-happy," Harvey said yesterday. "If these controversial weapons are being fired on a weekly basis in Afghanistan, we need to know that they are being used according to strict rules of engagement.

"Human rights groups have serious concerns about the effect of these weapons in populated areas, and their legality seems to be a grey area. The last thing we need in this counter-insurgency campaign is the allegation that civilians are dying at the hands of some kind of terror weapon. Parliament must be reassured these are a weapon of last resort."

A UK defence official told the Guardian that the Hellfire missiles that British Apaches had been initially equipped with were lighter anti-tank weapons. They would simply make a "small hole" in a building and the enemy would run away unscathed, the official said.

The new US-designed weapon was "particularly designed to take down structures and kill everyone in the buildings".

The official said British pilots' rules of engagement were strict and everything a pilot sees from the cockpit is recorded.