(Adds comments by U.S. special envoy for Iran, background)
By Jonathan Landay and Humeyra Pamuk
WASHINGTON, May 8 (Reuters) - The United States on Wednesday threatened to impose more sanctions on Iran 'very soon' and warned Europe against doing business with Tehran, increasing pressure on the Islamic republic hours after saying it will stop complying with parts of an international agreement to curb its nuclear program.
On the first anniversary of Washington's withdrawal from the 2015 landmark deal between Tehran and world powers, U.S. officials said they would keep squeezing Iran to force it to return to the negotiating table for a more comprehensive deal that would seek to curtail Iranian ambitions in the Middle East.
Tensions were already high between Washington and Tehran when the Trump administration said last weekend that it was deploying a carrier strike group and bombers to the Middle East, in response to what it said were 'troubling indications and warnings' from Iran.
Leading Democratic lawmakers such as Representative Adam Schiff warned that the Republican administration's moves could put the United States on the path to a war and House of Representatives Speaker Nancy Pelosi requested a briefing on Iran for members.
Hours before the U.S. threat of more sanctions, Iranian President Hassan Rouhani announced Tehran was reducing curbs to its nuclear program with steps that for now stopped short of violating the 2015 accord. But it threatened more action if countries did not shield it from sanctions.
Tehran halting compliance with some elements of the nuclear deal was "nothing less than nuclear blackmail of Europe," Tim Morrison, special assistant to the U.S. president and senior director for weapons of mass destruction, told a conference.
"Now is the time for the community of nations to strongly condemn Iran’s nuclear misconduct and increase pressure on the regime to comply with U.S. demands," Morrison said, adding that Washington was not 'done' with sanctions on Iran.
"Expect more sanctions soon. Very soon," he said.
Morrison said the United States would move quickly against any attempt by European countries to undermine Washington's sanctions pressure on Iran. He advised them against using the so-called Special Purpose Vehicle to facilitate non-dollar trade to circumvent U.S. sanctions.
"If you are a bank, an investor, an insurer or other business in Europe you should know that getting involved in the … Special Purpose Vehicle is a very poor business decision," Morrison said.
President Donald Trump a year ago pulled out of the accord signed by Iran, Russia, China, Britain, France, Germany and the United States to put limits on Iran's disputed nuclear program in return for the lifting of sanctions.
The Trump administration says the deal, negotiated by his predecessor Barack Obama, was flawed as it is not permanent, does not address Iran's ballistic missile program and does not punish it for waging proxy wars in other Middle East countries.
Spearheaded by national security adviser John Bolton, the Trump administration since then has taken several unprecedented steps to squeeze Iran such as demanding the world halt all Iranian oil imports and designating Iran's elite Revolutionary Guard Corps as a foreign terrorist organization, which Iran has cast as an American provocation.
"We have made our focus diplomatic isolation and economic pressure and that policy is working," Brian Hook, Special Envoy for Iran, said in a briefing.
Hook said that more nations now, compared with a year ago, were in agreement with the United States on Iran.
Washington's European allies opposed Trump's decision to withdraw from the nuclear deal and have failed so far to find ways to blunt the economic impact of new U.S. sanctions. France and Germany both said they wanted to keep the accord alive, and warned Iran not to violate it. (Reporting by Jonathan Landay and Humeyra Pamuk; Additional reporting by Patricia Zengerle, Editing by Mary Milliken and Grant McCool)