US President Donald Trump's decision on Tuesday to withdraw the country from an international nuclear deal with Iran has sparked strong reaction around the world, with Tehran and Brussels vowing to stand by the pact.

Trump said the United States was withdrawing from the 2015 Iran nuclear deal, which he said was "defective at its core". He also said in a televised address that he is planning to reinstate sanctions on Iran.

Iranian President Hassan Rouhani said that his country will remain in the nuclear deal with other signatories of the Joint Comprehensive Plan of Action (JCPOA) without the US.

The JCPOA was struck between Iran and the five permanent members of the United Nations Security Council — China, France, Russia, Britain and the US — plus Germany and the EU, in Vienna in 2015.

The international agreement, which limited Iran's nuclear program in return for sanctions relief, was negotiated under former US president Barack Obama in 2015.

"From this time on, the nuclear deal is an accord between Iran and five countries," Rouhani said. He said he has asked the Iranian foreign minister to initiate negotiations with the European partners as well as China and Russia over the future of the deal.

There was no immediate comment from China.

But early on Tuesday, Foreign Ministry spokesman Geng Shuang said, "Under the current circumstances, ensuring the integrity and sanctity of the JCPOA is conducive to upholding the international non-proliferation regime and promoting the peace and stability in the Middle East."

China will remain in close communication with all relevant parties and stay committed to upholding and implementing the JCPOA with an objective, impartial and responsible attitude, Geng told a regular news briefing in Beijing before Trump announced the exit.

UN Secretary-General Antonio Guterres said in a statement he was "deeply disappointed" by Trump's decision and urged the other signatories of the UN Security Council-endorsed pact "to abide fully".

Emma Ashford, a research fellow with the Cato Institute, a think tank in Washington, said in an analysis on Tuesday that by leaving the nuclear deal without offering any clear strategy or plan for an alternative, "Trump is opening Pandora's box, increasing the risks of escalation and bringing us gradually closer to conflict with Iran."

The Russian Foreign Ministry said in a statement that it was "deeply disappointed" by Trump's decision to unilaterally refuse to carry out commitments under the JCPOA.

"It is with regret and concern that we, the Leaders of France, Germany and the United Kingdom take note of President Trump's decision to withdraw," French President Emmanuel Macron, German Chancellor Angela Merkel and British Prime Minister Theresa May said in a joint statement.

European Union foreign policy chief Federica Mogherini said the bloc fully trusted the work, competence and autonomy of the International Atomic Energy Agency, which has published 10 reports certifying that Iran has fully complied with its commitments.

Israeli Prime Minister Benjamin Netanyahu expressed strong support for Trump's "bold" decision to "reject the disastrous nuclear deal".

Saudi Arabia also said it "supports and welcomes" Trump' decision.

Xinhua contributed to this story.