Reuters - European signatories to a nuclear pact with Iran said on Tuesday they were �extremely concerned� by Tehran�s apparent breach of the 2015 deal, as Israel said it was preparing for possible involvement in any confrontation between Iran and the United States.

Iran announced this week it has amassed more low-enriched uranium than is permitted under the nuclear pact, a move that prompted U.S. President Donald Trump to say Iran was �playing with fire�.

�We regret this decision by Iran, which calls into question an essential instrument of nuclear non-proliferation,� the foreign ministers of Germany, France and Britain said in a joint statement with the EU�s High Representative on Iran. �We urge Iran to reverse this step and to refrain from further measures that undermine the nuclear deal,� they said.

Tensions between Washington and Tehran have increased since Trump pulled Washington out of the pact last year and moved to bar all international sales of Iranian oil. Washington also blames Iran for attacks on oil tankers in the Gulf, something Tehran denies.

Iran announced this week it has amassed more low-enriched uranium than is permitted under the nuclear pact, a move that prompted U.S. President Donald Trump to say Iran was �playing with fire�.

�We regret this decision by Iran, which calls into question an essential instrument of nuclear non-proliferation,� the foreign ministers of Germany, France and Britain said in a joint statement with the EU�s High Representative on Iran. �We urge Iran to reverse this step and to refrain from further measures that undermine the nuclear deal,� they said.

Tensions between Washington and Tehran have increased since Trump pulled Washington out of the pact last year and moved to bar all international sales of Iranian oil. Washington also blames Iran for attacks on oil tankers in the Gulf, something Tehran denies.

The European signatories to the accord have sought to pull back the two longstanding foes from direct confrontation, fearing a mistake could lead to war accidentally.

Israel has encouraged the Trump administration to press ahead with sanctions against its arch-foe Iran, predicting that Tehran will eventually renegotiate a more limiting nuclear deal.

But Foreign Minister Israel Katz told an international security forum that Iran might accidentally stumble out of what he termed the �gray zone� of contained confrontation.

�It should be taken into account that mistaken calculations by the (Iranian) regime ... are liable to bring about a shift from the �gray zone� to the �red zone� - that is, a military conflagration,� he said in a speech to the Herzliya Conference.

�We must be prepared for this, and thus the State of Israel continues to devote itself to building up its military might for the event that it will have to respond to escalation scenarios.�

Israel has long threatened to take preemptive military action to deny Iran the means of making nuclear weapons. Tehran says it has no such designs. One of its senior lawmakers warned on Monday that Israel would be destroyed within �only half an hour� should the United States attack Iran.

Iranian Foreign Minister Mohammad Javad Zarif denies that Iran is in violation of the nuclear accord by amassing more low-enriched uranium, saying Iran is exercising its right to respond following the U.S. pullout.

By exceeding the limit, Tehran could prompt the return of all international sanctions on Iran but one European diplomat, asked if Europe would trigger a dispute resolution mechanism that is part of the accord, said:

�Not for now. We want to defuse the crisis.�