Some 68 search and rescue teams from 45 countries were on standby, but the United Nations was awaiting a green light from authorities in Japan to deploy, said Elisabeth Byrs of the UN Office for the Coordination of Humanitarian Affairs (OCHA).
US President Barack Obama spoke to Japanese Prime Minister Naoto Kan to offer to help "in any way possible", the Japanese Jiji agency reported.
In a statement, Obama said: "The United States stands ready to help the Japanese people in this time of great trial ... The friendship and alliance between our two nations is unshakeable, and only strengthens our resolve to stand with the people of Japan as they overcome this tragedy."
In the most concrete instance of immediate help, the US Air Force flew coolant to the Fukushima nuclear plant to help deal with a potentially dangerous breakdown of the cooling system, Secretary of State Hillary Clinton said.
Debris piles up after Tsunami (Photo: AP)
The Russian emergency services agency ERMACOM offered 40 people with three sniffer dogs, while Singapore had civil defence forces on standby and Poland offered firefighters.
China, Switzerland and the United States also offered rescue teams, while Britain, France and others said they were ready to offer whatever help was required.
"The world is shocked and saddened by the images coming out of Japan this morning," UN Secretary-General Ban Ki-moon told reporters in New York. "We will do anything and everything we can at this very difficult time."
Prime Minister Benjamin Netanyahu also offered aid, expressing Israel's condolences. President Shimon Peres sent a letter to Japan's leaders saying that "the citizens of Israel pray for Japan and its citizens".
"The Israeli government has expressed its willingness to send special forces and a professional aid delegation to support the rescue efforts," Peres added.
It was not clear whether Japan would in fact request foreign assistance as its emergency services and civil defence mechanisms are highly developed, according to aid officials in Geneva, the world's humanitarian hub.
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