BRUSSELS/PARIS (Reuters) - European powers will reaffirm on Thursday their support for the Iran nuclear deal that Donald Trump has rejected, EU diplomats said, on the eve of a deadline for the U.S. president to decide whether to reimpose oil sanctions lifted under the agreement.

At a meeting with Iran, Britain, France, and Germany, convened by the EU’s top diplomat Federica Mogherini, the European powers that helped negotiate the 2015 accord in Vienna will reassure Tehran they remain committed to it.

They will also urge Iran to continue to comply with international inspectors, the diplomats said. Tehran has always denied seeking nuclear arms.

“The aim is to send a message to Washington that Iran is complying and that it is better to have the nuclear agreement than to isolate Tehran,” one diplomat said.

A second diplomat said: “The date of the meeting is not a coincidence ... It’s a campaign that we have carried on since October,” referring to Trump’s decision not to certify that Tehran is meeting the terms of a pact to stop it developing nuclear weapons.

French Foreign Minister Jean-Yves Le Drian is to raise Paris’ concerns about the deal as well as its ballistic missile program while also seeking to keep dialogue open with Iran, a spokesman told journalists.

“France is determined to keep in place the Vienna accord... Iran must vigorously apply its engagements,” deputy foreign ministry spokesman Alexandre Giorgini said.

Slideshow ( 2 images )

A spokesman for Iran’s atomic energy agency said on Wednesday that a reimposition of sanctions by the United States would be a violation of the nuclear deal and added that the Islamic Republic had the capacity to greatly increase its enrichment of uranium.

The foreign ministers of Britain, Germany, France as well as Iran’s Mohammad Javad Zarif, are expected to meet on Thursday morning with Mogherini, the EU’s foreign policy chief.

The Thursday meeting in Brussels is part of diplomacy on both sides of the Atlantic before deadlines related to the deal falling this month, including deciding whether to reimpose oil sanctions lifted under the deal.

Trump must decide by mid-January whether to continue waiving U.S. sanctions on Iran’s oil exports under the terms of the pact. The State Department said on Tuesday the Trump administration was expected to decide on Friday.

The decision comes as Iran’s government deals with protests over economic hardships and corruption that are linked to frustration among younger Iranians who hoped to see more benefits from the lifting of sanctions.

Retired U.S. military officers, members of the U.S. Congress and former U.S. ambassadors were among 52 U.S. national security experts who signed a letter released on Monday urging Trump not to jeopardize the deal with Iran.

European countries including France and Italy have benefited from renewed trade with Iran, whose proven natural gas reserves are as vast as Russia’s, while Britain reopened its embassy in Tehran following the deal.

If Trump reimposes sanctions on Tehran, European powers fear the deal would fall apart. The EU is adamant the deal cannot be renegotiated.