The Minister for Foreign Affairs, Mr Cowen, has condemned the "seemingly endless cycle of violence" in the Middle East.
Speaking on behalf of the European Union, Mr Cowen said there was "deep concern" at the ongoing violence involving Palestinian groups and the Israeli Defence Forces.
He condemned yesterday's assassination of Hamas leader Abdel-Aziz al-Rantissi by Israeli forces and the suicide bombing at the Eretz crossing in Gaza earlier in the day and called for an immediate end to the violence.
Mr Cowen also said the Israeli government should remember that extra judicial killings are contrary to international law. "Respect for international law should mark an important distinction between democratically elected governments and terrorist groups," he said.
Mr Cowen said there the fighting was "futile" as a means of freeing Palestinians from occupation, or bringing long-term peace and security to Israelis. He also restated the European Union's position that a negotiated settlement is the only means for achieving a just and lasting peace.
Russia also denounced the killing, saying the escalation of tensions in the region could shatter hopes for peace. Foreign Ministry spokesman Alexander Yakovenko said in a statement: "Moscow is seriously concerned about possible consequences of that and the escalation of tensions in the Middle East."
"Such use of force cannot lead to a Palestinian-Israeli settlement, nor can it help a turn towards peace in the region as a whole." He said Russia acknowledged Israel's right to self-defence from terror attacks, but believed that it "must be exercised within the framework of international law."
In the US, the Bush administration declined to criticise Rantisi's killing, saying instead that Israel has the right to defend itself from terrorist attacks and urging Palestinians to use restraint in responding. "The United States strongly urges Israel to consider carefully the consequences of its actions," White House press secretary Scott McClellan said.
The Ireland-Palestine Solidarity Campaign also condemned the killing of Rantissi. "This assassination can only further inflame Palestinian and Arab anger against Israel and the United States, which was the only nation not to condemn the killing," an ISPC statement said.
The statement said that although EU foreign affairs chief Mr Javier Solana condemned the killing, "ritualistic condemnation is no longer an adequate response".
The ISPC called on the European Council and Commission to suspend trade privileges accorded to Israel by the EU. "Anything less amounts to complicity in Israel's contempt for international law and the will of the international community," it said.