The decision by Germany, France and Britain to back a UN arms embargo extension on Iran follows growing tensions with Tehran since US President Donald Trump unilaterally pulled out of the 2015 nuclear accord and introduced new sanctions.

The three European signatories to the Iran nuclear deal said they had reservations about lifting the arms embargo, a blow to Tehran which had been calling for an end to the restrictions.

The joint statement by three European foreign ministers came after the UN nuclear watchdog on Friday also passed a resolution critical of Iran -- the first since 2012 -- and demanded Tehran allow more inspections of two of its nuclear sites.

Iran agreed with major world powers in 2015 to halt its nuclear programme in return for an end to crippling sanctions. But since Trump pulled out of the deal, Iran has begun to roll back its own commitments, fuelling US criticism it wants to build nuclear arms, a claim Tehran denies.

The ban on selling weapons -- such as battle tanks, combat aircraft, warships and missiles or missile systems -- to Iran had been set to be progressively eased from October.

"We believe that the planned lifting of the UN conventional arms embargo established by Resolution 2231 next October would have major implications for regional security and stability," the European ministers said.

But the European nations, who remain committed to the nuclear deal, also said they opposed Washington's "maximum" pressure tactics with sanctions.

"We firmly believe that any unilateral attempt to trigger UN sanctions snapback would have serious adverse consequences" in the UN Security Council, they said.

Iranian President Hassan Rouhani earlier this month urged UN Security Council members to oppose a US "conspiracy" to extend the arms embargo.

The three European powers said they plan to address the arms embargo issue "in close coordination" with UN Security Council permanent members Russia and China.

- Need for inspections -

The International Atomic Energy Agency (IAEA) earlier on Friday passed a resolution put forward by European states, urging Tehran to provide inspectors access to two sites in Iran to help clarify whether undeclared nuclear activity took place there in the early 2000s.

It calls on Iran to fully cooperate with the IAEA and satisfy its requests without delay, including by providing prompt access to the sites.

Iran has been blocking access to the sites for months, prompting a growing diplomatic row.

US Secretary of State Mike Pompeo, who has advocated a hard line against Iran on multiple fronts, said Tehran must "immediately comply" or face consequences.

"If Iran fails to cooperate, the international community must be prepared to take further action," he said in a statement.

But Iran's envoy to the UN in Vienna, Kazem Gharib Abadi, insisted the resolution will not "encourage Iran to grant access to the Agency based on fabricated and unfounded allegations".

"Iran categorically deplores this resolution and will take appropriate action in response," he said.

Even though the sites in question are not thought to be directly relevant to Iran's current nuclear programme, the agency says it needs to know if activities going back almost two decades have been properly declared and all materials accounted for.

- 'No exceptions' -

Iran has hinted that such an IAEA resolution could cause "complication and difficulties" for the future of the 2015 accord, also known as the Joint Comprehensive Plan of Action (JCPOA).

Speaking to reporters after the resolution was passed, IAEA Director General Rafael Grossi said it would be "absolutely unacceptable" if an example were to be set that states can be selective in their implementation of agreements with the UN agency.

"There are no exceptions," Grossi said. "I intend to sit down with Iran very soon and to try to solve this as soon as possible."

Iran says the IAEA's access requests are based on allegations from Tehran's arch-enemy Israel.

Additional information provided by the IAEA in the form of satellite images to back up its requests "contained no convincing underlying reason" to provide access, Tehran argues.