By Laurence Norman 

VIENNA -- France, Britain and Germany will ratchet up pressure on Iran in coming weeks by triggering a dispute mechanism if Tehran continues its prohibited moves away from the 2015 nuclear deal, diplomats say.

The warning leaves the two sides heading for a major clash in early January, when Iran has said it will further escalate its nuclear program. Advancing with the dispute mechanism, part of the 2015 nuclear deal, has prompted Iranian threats of abandoning the deal and could lead to the reimposition of international sanctions on Tehran within two months.

The European warning comes as senior Iranian, European, Russian and Chinese officials meet in Vienna on Friday to discuss the threats to the 2015 deal. The U.S. withdrew from the accord in May 2018 and has since imposed withering sanctions on Tehran.

Iran's breaches of the accord came in response to those decisions and the failure of Europe to cushion the Iranian economy from the impact of U.S. sanctions.

While no final European decision has yet been taken, diplomats say the thinking on how to respond to Iran's nuclear steps has shifted significantly in recent weeks. As Iran has made increasingly bold moves away from the deal, the perceived cost of inaction has risen. The Trump administration has repeatedly urged Europe to pressure Iran over its deal violations.

European officials were hesitant to take any step as long as French-led efforts to engineer U.S.-Iranian talks on de-escalating tensions were possible. But the prospect of the negotiations happening soon appears to be almost zero, they say.

Some officials believe Tehran may be overestimating its ability to push Washington into easing economic sanctions by threatening to scale up regional and nuclear tensions ahead of the 2020 U.S. presidential election. While President Trump has repeatedly said he wants to avoid a fresh Middle East conflict and is keen on talks with Iran, U.S. officials say the administration's maximum-pressure sanctions campaign is devastating the Iranian economy and straining the regime. European officials say it may become harder for Mr. Trump to cut a deal with Tehran, which could prove a difficult sell with his Republican party, as the election approaches.

The European threat comes as a wave of protests in Iran has been met with a bloody response from the regime, leaving hundreds dead. Iran's Supreme Leader Ayatollah Ali Khamenei, the ultimate decision maker on major security issues, has warned against further diplomacy with the West, saying in September that the Europeans were untrustworthy, offering only "empty words and promises."

While many European countries remain committed to the nuclear deal, relations between Europe's big powers and Iran have been souring for months.

European powers have joined Washington in blaming Iran for attacks on oil tankers in the Gulf and on major Saudi oil installations. They have criticized Iran's response to the protests, this week denounced Tehran's ballistic missile program work and have grown more alarmed about Iran's role in Iraq, Lebanon and other neighboring countries.

The triggering of the dispute mechanism wouldn't bring the nuclear deal to an automatic end. Instead, it would begin a weekslong process of negotiations aimed at pressing Iran back into compliance with the deal, at the end of which the matter could be escalated to the UN Security Council. There, a decision to reimpose international sanctions on Iran could be taken by just one of the five permanent UN Security Council members, including the U.S.

European diplomats frame the decision to trigger the dispute mechanism as a new phase of diplomacy aimed to shore up the deal by escalating pressure on Tehran to come back in line with its commitments. They stress that they will not rush the process, noting that it allows for repeated delays for longer discussions and could run on for months if need be.

However, the risks of taking that step are real. Some European officials believe it could lead Iran into a further escalation of its nuclear work. Others believe Iran would only withdraw from the deal if international sanctions are reinstated.

In recent weeks Iran has bristled at European threats to activate the dispute procedure. A top Iranian nuclear negotiator said last month the decision would cross Iran's red line. After France's foreign minister said publicly that Europe was considering triggering the mechanism, Iran's Foreign Minister Javad Zarif accused him of issuing "arrogant threats."

Iran since May has carried out its warning that it would suspend some of its commitments under the nuclear deal every two months. During that time, it has significantly accelerated its build up of enriched uranium, going over the deal's limits on the amount it can stockpile. It has also expanded research work on producing much more powerful machines for enriching uranium beyond agreed limits in the deal.

It has resumed enriching uranium at its underground nuclear facility at Fordow, and it is enriching uranium to a purity above what the deal allowed. It was also accused by the U.S. of detaining a weapons inspector and is under pressure to explain traces of uranium found at a site in Tehran.

Nonetheless, Western officials believe Iran is still many months from amassing enough enriched uranium for a nuclear weapon. The enriched uranium it is producing, which has a purity of up to 4.5%, is also far below the roughly 90% levels needed for weapons grade material.

The protests in Iran in recent weeks have likely further narrowed Tehran's appetite to pause its nuclear escalation, observers say, weakening the position of President Hassan Rouhani's government, which sparked the protests by removing fuel subsidies.

Write to Laurence Norman at laurence.norman@wsj.com

(END) Dow Jones Newswires

December 06, 2019 06:50 ET (11:50 GMT)