US President Donald Trump extended a sanctions waiver on Iran on Friday, keeping the landmark nuclear deal with Tehran in force for at least another four months.

Trump did, however, warn that Friday's waiver would be the last time he was prepared to prevent Congress from putting sanctions in place, unless a significantly strengthened agreement was reached with Iran and the United States' European allies within 120 days.

Read more: Iran nuclear deal relies on 'full compliance' from US

"This is a last chance," Trump said in a statement. "In the absence of such an agreement, the United States will not again waive sanctions in order to stay in the Iran nuclear deal. And if at any time I judge that such an agreement is not within reach, I will withdraw from the deal immediately."

US Congress requires the president regularly recertify that Iran is in compliance with the agreement — a deal Trump has called the worst the US has ever entered into and which he promised to "rip up."

Trump said he wants to amend US law to make Iran's long-range missile program inseparable from its nuclear program.

The White House noted that a new deal would also be permanent and not expire after 10 years, as is the case under the current 2015 accord.

Following the president's announcement, Iran's Foreign Minister Mohammad Javad Zarif accused Trump of desperately wanting to undermine the deal, saying the accord is "not negotiable."

Germany to 'remain committed'

Late Friday, the German Foreign Ministry said it had taken "note" of Trump's announcement and would respond after discussing the matter with "our European partners."

"We remain committed to the continued full implementation of the agreement," it said in a tweet.

The EU and UK also acknowledged Trump's announcement, but said they would not decide on a course of action until discussing it further with the deal's European signatories.

However, an advocacy group established by former US Secretary of State John Kerry was more confrontational, saying Trump's threat to withdraw undermines the country's credibility with its allies.

"A US move to violate or withdraw from the nuclear deal undermines the trans-Atlantic relationship and our alliances of first resort, which are vital across a broad range of policies — from bedrock economic issues to confronting terrorism," the group said.

US Treasury sanctions Iranian officials

The White House's announcement coincided with a series of new sanctions by the Treasury Department targeting Iranian officials. Among them is the head of Iran's judiciary, Sadegh Amoli Larijani. The White House said the move was a response to Tehran's crackdown on recent anti-government demonstrations, which left 18 protesters dead and thousands under arrest.

Read more: Ayatollah Khamenei slams US 'bullying' Iran on nuclear deal

The new sanctions come on top of previous designations announced back in May and October. Those mainly targeted Iranian defense officials, as well as a China-based network the US says is supporting Iran's missile program.

ls, dm/cmk (AP, Reuters, AFP, dpa)