WASHINGTON (Reuters) - The ambassadors to Washington from Britain, France, Germany and the European Union all strongly backed the international nuclear agreement with Iran on Monday, as long as Tehran continues to comply with the pact. U.S. President Donald Trump is weighing whether the 2015 deal serves U.S. security interests as he faces a mid-October deadline for certifying that Iran is complying with the pact, a decision that could sink an agreement strongly supported by the other world powers that negotiated it.  We agree that the demise of this agreement would be a major loss,  David O Sullivan, the European Union s envoy in Washington, said at an Atlantic Council panel discussion. German Ambassador Peter Wittig said anyone advocating walking away should consider  larger issues,  including an increased danger Iran would resume enrichment, danger of a nuclear arms race in an unstable region and impact on global nonproliferation efforts.  What kind of signal would this send to countries like North Korea?  Wittig asked.  It would send a signal that diplomacy is not reliable, that you can t trust diplomatic agreements, and that would affect, I believe, our credibility in the West when we re not honoring an agreement that Iran has not violated.  If Trump does not recertify by Oct. 16, Congress has 60 days to decide whether to reimpose sanctions suspended under the accord. That would let Congress, which is controlled by Trump s fellow Republicans, effectively decide whether to kill the deal. Although congressional leaders have declined to say whether they would seek to reimpose sanctions, every Republican lawmaker opposed the deal reached by Democratic President Barack Obama s administration. Many, like Trump, have made opposition to the agreement a campaign issue. If Washington pulls out of the deal, the ambassadors said they would do everything possible to protect any companies based in Europe that continue to do business with Iran from reimposed U.S. sanctions.    Britain s ambassador, Kim Darroch, said Trump and Prime Minister Theresa May had devoted about half their discussion to Iran when they met in New York last week, although Trump did not reveal his decision. He said May had explained again why Britain supports the nuclear pact, seeing it as a matter of national security.  As long as the Iranians continue to comply with it, in the view of the IAEA, we will continue to support it,  he said, referring to the International Atomic Energy Agency. Speaking separately at another event in Washington, White House national security adviser H.R. McMaster defended Trump s criticism of the deal.  I obviously agree with the president on this, I think it was the worst deal. It gave Iran all of the benefits up front,  McMaster said, adding that it had the  fatal flaw of a  sunset clause .  He was speaking at an event hosted by the Institute for the Study of War. The so-called sunset clauses are provisions under which some of the deal s restrictions on Iran s nuclear program expire from 2025. French Ambassador Gerard Araud noted that the other countries that signed the pact - Russia, China and Iran - had made clear that they do not support renegotiating. 