Jack Straw led international condemnation today of Israel's assassination of Sheikh Ahmed Yassin, Hamas's founder and spiritual leader, in a dawn air strike.

"It is unacceptable, it is unjustified and it is very unlikely to achieve its objective," the Foreign Secretarytold reporters as he arrived in Brussels for European Union talks on tackling terrorism.

He called for restraint on both sides: "All of us understand Israel's need to defend itself against terrorism which affects it - within international law." But he said that Israel was not entitled to carry out "this kind of unlawful killing, which we condemn".

Mr Straw said that he did not believe that Israel would benefit from the killing of an old man in a wheelchair.

He later appealed to the Arab world for calm.

Tony Blair's official spokesman condemned the attack as unlawful.

"What has happened this morning is clearly a setback -- there is no point pretending otherwise," the spokesman said.

"We have repeatedly made clear our opposition to Israel's use of targeted killings and assassinations. We recognise Israel's right to defend itself against terrorism, but equally any steps should be within international law and should be neither disproportionate nor excessive."

Kofi Annan, the UN Secretary-General, also called into question the legality of the strike.

He said: "I do condemn the targeted assassination of Sheikh Yassin and the others who died with him. Such actions are not only contrary to international law, but they do not do anything to help the search for a peaceful solution.

"I appeal to all in the region to remain calm and avoid any further escalation in tensions," he said.

The Vatican said that the attacks would only lead to a cycle of violence that would not further the cause of peace in the Middle East.