Japan and France Thursday pushed Iran to allay international suspicions about its nuclear programme, saying the time has come for Tehran to reject suggestions that it might be developing atomic bombs.

Japanese Foreign Minister Katsuya Okada and his French counterpart Bernard Kouchner said they "share serious concerns" about Iran's nuclear programme, as the US and European nations mount pressure for new sanctions on Tehran.

"If Iran were to hold nuclear weapons, the seriousness of the matter is immeasurable," Okada told a joint press conference after his meeting with Kouchner.

"I said Iran has no more time left. They have to decide now," he told the press conference.

Japan must play a sensitive balancing act, as the resource-pour nation relies heavily on Iran for its energy supplies. In a rare break with the United States, its main ally, Japan has maintained warm ties with Iran.

France, the United States and others however are stepping up efforts to rally support for fresh United Nations sanctions on Iran, which key world powers suspect is trying to make a nuclear weapon.

The Islamic Republic insists its uranium-enrichment activities are aimed at generating power for civilian use.

Kouchner said many nations have tried to engage Iran but have not been successful, adding that the international community must take more decisive steps in dealing with Iran.

Okada, stressing that he respected Iran's culture and history, said Tokyo "will have to play an important role" when it becomes president of the UN Security Council in April.

"If the Security Council approves a new resolution, we will not hesitate to implement it," he said.

Japan has received frequent international calls to play a more assertive role in pushing Iran over its nuclear programme.

US Deputy Secretary of State Jim Steinberg visited Japan earlier this month and told Okada that Tokyo would have to play "a very critical role" to limit Iran's nuclear ambitions.

Among other topics discussed with Kouchner, Okada said he dealt with Japan's desire to sign a global treaty on international parent-child abductions.

Until Japan signs the agreement, Tokyo will prioritise allowing meetings between children and their divorced parents, he said.

Japan is the only country among the Group of Seven industrialised nations that has not signed the 1980 Hague Convention that requires countries to return a child wrongfully kept there to their home country.

Activists say thousands of foreign parents have lost access to children in Japan, where the courts virtually never award custody to a divorced foreign parent.