German chancellor Angela Merkel has led a number of world leaders in expressing disappointment over President Donald Trump's decision to pull the US out of a nuclear deal with Iran.
She said the US withdrawal shows Europe will face increasing responsibility to secure peace and seek political solutions to conflicts.
Merkel underlined the commitment of Germany, France and the UK to stick with the accord.
On Wednesday she told members of her conservative party: "We have taken note with regret but also concern of this withdrawal by the United States of America ... We will remain committed to this agreement and try to do everything so that Iran also fulfils its commitments in the future."
She added: "Yesterday showed us once again that we will face more responsibility in Europe, in foreign policy, in the area of securing peace, in the area of the political solutions we must find."
China also expressed regret over the US withdrawal, but reaffirmed its commitment to the landmark pact.
Foreign ministry spokesman Geng Shuang told reporters that "ensuring the integrity and sanctity" of the agreement was important for upholding the international non-proliferation regime and promoting peace and stability in the Middle East.
China was involved in negotiating the agreement as one of the five permanent members of the United Nations Security Council and has long been a close Iranian economic partner, buying about a third of Iran's oil shipments
France and the European Union have pledged to work with the Trump administration to ensure European business interests in Iran are protected in the wake of the withdrawal.
An adviser to French president Emmanuel Macron said the US decision to re-impose sanctions on Iran means European companies could be affected.
An adviser said: "We are going to do everything with the businesses involved to safeguard our interests. We will have discussions at the European level."
He added that France wants to preserve the Iran nuclear deal as part of its effort to guarantee global security in the Middle East, and that Macron would speak with Iranian president Hassan Rouhani later.
Israel, Saudi Arabia and Iran's other rivals have long wanted to scuttle the nuclear deal, which they see as undermining the strategy they say the world should be taking: a tough, confrontational stance against Tehran's ambitions in the region.
But the deal's unravelling could backfire and spark even more unrest in the Middle East. Also, if Iran follows with an all-out revival of its nuclear program, Saudi Arabia has threatened to launch a nuclear weapons program of its own in response.
Israeli PM Benjamin Netanyahu has bluntly said the world would be better off without any deal than with what he calls the "fatally flawed" agreement reached during the Obama administration.
Meanwhile, the head of the UN's nuclear watchdog has said Iran is fulfilling its commitments under the nuclear deal with world powers.
The International Atomic Energy Agency's director-general, Yukiya Amano, said that "as of today, the IAEA can confirm that the nuclear-related commitments are being implemented by Iran".