Iran is actively trying to buy nuclear technology through blacklisted companies, according to a confidential UN report, citing information from the British government.

The claims -- which if true would violate UN sanctions -- were made to a UN panel of experts just weeks after world powers reached a framework deal with Iran on curbing its nuclear program.

"The UK government informed the panel on the 20 April 2015 that it 'is aware of an active Iranian nuclear procurement network which has been associated with Iran's Centrifuge Technology Company (TESA) and Kalay Electric Company (KEC)'," the report, seen by AFP, said.

"Given the late communication, the panel could not independently investigate the above information."

The United Nations has slapped a series of sanctions on Iran over its failure to address international concerns about its nuclear program and suspicions that it could have military purposes.

KEC is on a UN list of blacklisted Iranian companies for its ties to the nuclear program.

The report was presented to the Security Council's Iran sanctions committee on April 21, just as negotiators from the P5+1 -- Britain, China, France, Russia, the United States plus Germany - began work on finalizing the nuclear deal.

The panel noted that overall there had been few reported violations of UN sanctions.

"It might be linked, inter alia, to a decrease in the Islamic Republic of Iran's prohibited activities and restraint on the part of member-states so as not to affect the negotiations process" for the nuclear deal, the report said.

World powers have set June 30 as the deadline for finalizing the agreement that would end one of the most vexing disputes in international diplomacy and could open the door to revamping relations with Iran.