South Korea and Singapore have intercepted suspected nuclear and weapons materials bound for Iran that breach UN sanctions imposed on the Islamic republic, diplomats said Thursday.

The two seizures, made in the past six months but only revealed now, add to a growing list of alleged Iranian attempts to breach an international arms embargo, which are bringing mounting pressure to tighten sanctions, they said.

"South Korea authorities found more than 400 suspicious tubes in a jet cargo at Seoul airport in December," one diplomat told AFP, giving details from a report to the UN Iran sanctions committee.

The tubes could be used for nuclear facilities, the diplomat said, speaking on condition of anonymity because the seizures have not been made public.

"In September, aluminium powder that can be used for rockets was found on a ship in Singapore harbor," the diplomat added.

In each case the product was destined for Iran.

The details were confirmed by a second envoy at the UN who said the sanctions committee would study them when it discusses the latest report from Iran experts monitoring the sanctions regime.

"This is going to add to calls for at least tighter implementation of the current sanctions," said the envoy, also speaking on condition of anonymity. "There are still too many countries not applying the measures."

The United Nations has passed four rounds of sanctions against Iran for its refusal to halt uranium enrichment, including a global arms embargo.

Western nations accuse Iran of seeking a nuclear bomb while Iran insists its enrichment program is for purely civilian purposes.

Over the past year a growing number of cases of weapons and explosives seizures involving Iran have been brought before the UN sanctions committee.

Up to seven tons of RDX high explosive were found on a ship in an Italian port in September that was going from Iran to Syria.

And Nigerian agents seized 13 containers of weapons, including rockets and grenades, in Lagos port in October.

The containers were loaded at the Iranian port of Bandar Abbas and were reportedly destined for separatist rebels in Senegal's Casamances region. Senegal has since broken diplomatic ties with Tehran.

Britain said this month that arms seized by its special forces in Afghanistan on February 5 were being supplied by Iran to the Taliban militia fighting international forces.

British Foreign Secretary William Hague condemned what he called Iran's "completely unacceptable" actions.

The pressure will likely increase after Israel this week accused Iran of supplying weapons found on a ship intercepted by naval commandos as it headed for the Hamas-run Palestinian territory of Gaza. Iran has denied the claim.

Israeli authorities showed off the weapons on Wednesday, and Prime Minister Benjamin Netanyahu said they were "destined for terror groups in the heart of Gaza".

"The source of the weapons was Iran, which is trying to arm the Strip," he added.

The Israeli army said there were about 50 tons of weapons on the ship. They included Chinese-designed C704 anti-ship missiles with a range of up to 35 kilometers (20 miles) as well as ammunition and mortars.

There was also an advanced radar system to guide the missiles.

The weapons were hidden in containers labelled lentils and cotton, the army said. The explosives found in Italy were described on documents as a consignment of powdered milk.

At the last Security Council debate on the Iran sanctions, Britain called for new Iranian individuals and entities to be added to sanctions lists. France said there should be a detailed investigation of Iran's sanctions-busting.