New Delhi, Nov. 11: Prime Minister Narendra Modi today oversaw the signing of a landmark nuclear deal with his Japanese counterpart Shinzo Abe after New Delhi made an unprecedented bilateral commitment of its intent to not conduct nuclear tests in future.

The pact, which now needs to be ratified by Japan's Parliament - known as the Diet - before it can be enforced, will allow India to source technology directly from Japan, and will eliminate a key challenge in the implementation of the Indo-US nuclear deal.

American nuclear power giant Westinghouse, which plans to set up six reactors in India, is owned by Japanese firm Toshiba. US company GE, which too is keen to set up reactors in India, operates its nuclear technology arm in conjunction with Hitachi, also a Japanese firm.

But a separate written commitment India has given Japan - apart from the nuclear deal - explicitly iterates the country's unilateral moratorium on tests declared after the 1998 nuclear explosions, two officials told The Telegraph.

The nuclear deal signed by the two nations today includes a clause that allows Japan to withdraw from the pact after giving India a year's notice.

That a nuclear test by India would lead to a nullification of the agreement with Japan - and similar pacts with many other countries - was never in doubt for New Delhi.

But India, which announced a unilateral moratorium on tests after the 1998 nuclear explosions, has never in the past agreed to the explicit detailing of such a condition while signing nuclear agreements with other countries, including the US, France, Russia, Canada and Australia.

Japan's history as the only country to have suffered from nuclear explosions made the bilateral concession necessary, officials said.

It is unclear if the memorandum also explicitly states that Japan will withdraw from the nuclear agreement if India does carry out tests.

"Today's signing of the Agreement for Cooperation in Peaceful Uses of Nuclear Energy marks a historic step in our engagement to build a clean energy partnership," Modi said in a statement after his meeting with Abe. "Our cooperation in this field will help us combat the challenge of climate change."

India has traditionally feared that such explicit detailing of a link between nuclear cooperation and future tests could fuel similar demands from other countries. A multiplicity of such demands could effectively turn a unilateral promise to not conduct nuclear tests into a treaty commitment.

With every country till now, India has settled for agreements that - as with the Japan pact - allow the other country to withdraw from the pact after giving a notice but without any explicit reference to nuclear tests.

Foreign secretary S. Jaishankar, in a late-evening media briefing in Tokyo, contended that the separate commitment on maintaining the moratorium on testing - while unprecedented in India's bilateral agreements - wasn't completely new.

In 2008, when India sought a special waiver from the nuclear suppliers' group (NSG), New Delhi had iterated its commitment to the moratorium, said Jaishankar, who was a key negotiator of the Indo-US nuclear deal.

India's application for full membership of the NSG - which the cartel of nuclear trading nations is considering - also iterates this commitment, he said.

"So, it really isn't something very new or unique," Jaishankar argued. "That commitment of September 2008 was reiterated in the context of the agreement today."

Nor is New Delhi the only one to have made a concession, officials pointed out.

Japan has civil nuclear agreements with 13 countries - but never before with a country that is not a signatory to the non-proliferation treaty (NPT). India has refused to sign the NPT, arguing that it is discriminatory.

For both countries, the nuclear pact represents a key step towards their growing strategic convergence - marked by shared security concerns in the Asia Pacific because of an increasingly muscular China, and joint economic projects.

On Friday, Modi and Abe iterated their commitment to deepening security cooperation, including through joint maritime exercises, and sent a message to China by urging all nations to respect the UN Convention on the Law of the Seas (UNCLOS).

China has refused to accept an UNCLOS tribunal's verdict against it on a maritime dispute with the Philippines.

Jaishankar said the two countries also agreed on a timeline for the construction of the first bullet train in India - between Mumbai and Ahmedabad. The train will be running by 2022, he said, and Modi and Abe could inaugurate the groundwork for the project next year.

Modi also thanked Abe for his commitment to supplying US-2 amphibious aircraft to India, though the two countries are still negotiating costs and other specifications.

But the nuclear pact inked today - the text had largely been finalised last year when Abe visited India in December - is a cement that Indian officials have long been arguing is key to bolstering the partnership.

Asked if the nuclear agreement had a clause allowing Japan to withdraw, Jaishankar said it did - but so did many others among India's nuclear agreements.

"The one we have with the US also has a clause allowing a party to withdraw or secede from the agreement," Jaishankar said.

But the US agreement does not refer to Indian nuclear tests as a reason for the abrogation of the treaty.

One official accepted that India was also under pressure - Modi and US President Barack Obama had announced earlier this year that they would finalise the commercial agreements between the Indian government operator and Westinghouse by the summer of 2017. The Japan pact is critical for those commercial agreements.

"I also acknowledge the special significance that such an agreement has for Japan," Modi said, in his statement after the meeting with Abe.

India was aware that the Japanese Diet would not find it easy to approve the nuclear agreement without some concession.

But by Friday evening, Indian officials were exuding confidence that the Japanese parliament would ratify the pact.

"We all know that Japan has some sensitivities," Jaishankar said.

"But when the government of Japan concludes an agreement, it is with the expectation that it will be ratified."