George Bush, the US president, did not condemn Israel's attack on Beirut's international airport on Thursday.
After talks with Angela Merkel, the German chancellor, he told a news conference: "Israel has the right to defend herself."
But he warned the Israelis they should be careful not to weaken the fragile Lebanese government.
Bush also urged Syria to put pressure on Hezbollah to release the Israeli soldiers, saying: "Syria needs to be held to account."
Merkel called for a "de-escalation" of the conflict and said: "The attacks did not start from the Israeli side but from Hezbollah's side."
But Russia and the European Union said there could be no justification for Israel's air and sea blockade on Lebanon.
"This is a disproportionate response to what has happened "
Russian foreign minister
"Actions, which are contrary to international humanitarian law, can only aggravate the vicious circle of violence and retribution," the EU presidency said in a statement.
The comments came as a three-strong UN team headed to the Middle East in an attempt to defuse the crisis.
Sergei Lavrov, the Russian foreign minister, denounced Israel's attack on Lebanon and its operations against the occupied Palestinian territories.
"This is a disproportionate response to what has happened, and if both sides are going to drive each other into a tight corner, then I think that all this will develop in a very dramatic and tragic way," Interfax news agency reported him as saying.
French Foreign Minister Philippe Douste-Blazy called Israel's bombardment of the Beirut airport "a disproportionate act of war", saying there was a risk of a regional war.
The attacks drew criticism from Arab and Muslim nations, with Saudi Arabia on Thursday blaming "elements" inside Lebanon for the situation, seemingly criticising Hezbollah and Iran.
"A distinction must be made between legitimate resistance and uncalculated adventures undertaken by elements inside [Lebanon] and those behind them, without recourse to the legal authorities and consulting and co-ordinating with Arab nations," a statement on the official news agency SPA said.
Malaysia, the current leader of the 57-nation Organisation of the Islamic Conference, condemned Israel and urged other nations to take action.
Meanwhile, Iran warned Israel against attacking Lebanon's neighbour Syria.
"If Israel commits another act of idiocy and aggresses Syria, this will be the same as an aggression against the entire Islamic world and it will receive a stinging response," Mahmoud Ahmadinejad, the Iranian president, said in a telephone conversation with his Syrian counterpart Bashar al-Assad.
Ahmadinejad says Iran would be
at Lebanon's service
Ahmadinejad also told Emile Lahoud, the Lebanese president, that "Iran would put all its potential at the service of Lebanon".
Iran rejected claims that the two Israeli soldiers captured by Hezbollah on Wednesday were being moved there.
On Thursday, Israel said it had information that Hezbollah was trying to transfer them to Iran in an apparent move to prevent them from being rescued.
Mark Regev, an Israeli foreign ministry spokesman, did not disclose the source of his information.
Hamid Reza Asefi, the Iranian foreign ministry spokesman, said: "This regime (Israel) is trapped in its homemade crisis and these sort of accusations are simply nonsense."
UN emergency session
On Friday, the United Nations Security Council is holding an emergency session to discuss the Israeli offensive.
Lebanon has asked the UN to adopt a resolution demanding a ceasefire and an end to the Israeli attacks.
Israel told the UN on Wednesday that it held Lebanon responsible for the capture of the two soldiers, which it termed an act of war.
Elsewhere, the International Committee of the Red Cross voiced concern about the plight of civilians caught up in the conflict and urged all sides to respect international law.
The violence is the worst between Israel and Lebanon since 1996 when Israeli troops still occupied part of the south.