LONDON, May 29 (Reuter) - Britain said it was sending more troops to Bosnia to strengthen the ability of its U.N. peacekeeping units to defend themselves after Bosnian Serb forces seized more than 30 British soldiers as hostages.

"The Ministry of Defence will be sending two artillery batteries and an armoured engineer squadron to Bosnia as soon as possible," the government said in a statement after an emergency meeting of ministers and defence chiefs.

The 24 Airmobile Brigade, a high-speed anti-tank force, is also being placed under orders to prepare for deployment in Bosnia, in consultation with the United Nations.

The annoucement meant that up to 6,000 extra men with associated hardware could be sent.

Britain said it was consulting its NATO allies and other countries about further specific steps. It called for renewed diplomatic efforts to end hostilities in Bosnia and pre-empt a "further downward spiral of the war."

"We shall be proposing to our partners and allies that these additional deployments should form part of a wider strategy of concentrating the forces of the United Nations," it added.

British newspapers said Prime Minister John Major decided against a withdrawal of British troops during a 30-minute conversation with French President Jacques Chirac on Sunday.

Major requested the emergency talks, which a spokesman described as a "stock-taking meeting," before a U.N. spokesman in Sarajevo said Bosnian Serbs had taken 33 British U.N. soldiers hostage in eastern Bosnia.

The Serbs have surrounded and detained 367 U.N. soldiers and unarmed observers since NATO warplanes hit a Serb ammunition dump on Thursday and Friday. At least 17 were being held as human shields at potential targets to prevent more NATO strikes.

"The action of Bosnian Serb forces in taking and holding U.N. peacekeeping troops of different nationalities is without a shred of justification and has created a very serious situation," the British government said.

"It is an outrageous action against peacekeepers which the British government utterly condemns. The consequences will be very severe if these soldiers are not released unharmed."

The separatist Serbs warned U.N. officials that the safety of the Britons could not be ensured unless four Serbs captured in a firefight with French U.N. soldiers were released, U.N. sources in Bosnia said.

The government said it would seek the recall of parliament on May 31 to consider the situation in Bosnia. Parliament, now in recess, had not been due to reconvene until June 6.

Leaders of the opposition Labour Party also condemned the taking of the hostages.

"What is happening at the present time is a war crime and we should make it clear that we will hold the leaders personally responsible if any injury occurs to these people," Labour Party foreign affairs spokeman Robin Cook told BBC televison.

"It is vital that the international community remains resolute. If we back down against this blackmail we will have no authority in Bosnia or anywhere else we mount apeacekeeping operation."

Conservative Sir Nicholas Bonsor, chairman of parliament's defence select committee, said Britain should consult Major-General Rupert Smith, the British commander of U.N. forces in Bosnia, about the safety of the troops.

"I think we must take advice from the general as to whether he feels it is safe for us to continue to operate or whether in his judgment we need to withdraw," he said.