The flag of the International Atomic Energy Agency (IAEA) flies in front of its headquarters in Vienna, Austria, May 28, 2015.

:: The 2015 nuclear deal between Iran and world powers faces a stern test at the United Nations this week as Europeans try to persuade a skeptical Trump administration to keep it, while Israel lobbies to turn up the pressure on its regional rival.

U.S. President Donald Trump, who must make a decision by mid-October that could undermine the agreement, repeated on Thursday his long-held view that Iran was violating “the spirit” of the deal under which Tehran got sanctions relief in return for curbing its nuclear program.

The Republican president has called the agreement, struck under his Democratic predecessor, Barack Obama, “the worst deal ever negotiated.”

The prospect of Washington reneging on the agreement has worried some of the key U.S. allies that helped negotiate it, especially as the world grapples with another nuclear crisis, North Korea’s nuclear and ballistic missile development.

“We all share U.S. concerns about Iran’s destabilizing role in the region, but by mixing everything up, we risk losing everything,” said a senior European diplomat, who was part of the 18-month negotiation process that led to the accord.

Trump must decide in October whether to certify that Iran is complying with the agreement, known as the Joint Comprehensive Plan of Action (JCPOA). If he does not, Congress has 60 days to decide whether to reimpose sanctions waived under the deal.

Iran’s Supreme Leader Ayatollah Ali Khamenei warned on Sunday that Tehran would react strongly to any “wrong move” by Washington on the nuclear deal.

At the U.N. General Assembly on Monday, Trump meets Israeli Prime Minister Benjamin Netanyahu followed by French President Emmanuel Macron, who like Trump, is making his inaugural appearance at the annual gathering of world leaders.

Both have very different messages to deliver.

“Our position is straightforward. This is a bad deal. Either fix it — or cancel it. This is Israel’s position,” Netanyahu said in Argentina last Tuesday as he toured Latin America.

Israeli officials said he would also relay concerns over what Israel describes as Tehran’s growing military entrenchment in Syria and its post-civil war role in that country.

They said changes that Israel was seeking in JCPOA included lengthening the 10-year freeze on Iran’s nuclear development program or even making that suspension permanent and destroying centrifuges rather than temporarily halting their operation.