The two-day talks between Iran and the U.N. nuclear agency failed to arrive at a consensus on resuming a stalled investigation into the Islamic state’s suspected nuclear weapon facilities, Reuters reported, citing diplomatic sources.

The International Atomic Energy Agency, or IAEA, and Iran apparently differed on key issues during the talks that ended Thursday, mounting the concern over the country’s nuclear plans.

The Vienna-based nuclear watchdog had conducted many rounds of talks with Iran to convince the state to open up its key nuclear installations for scrutiny by the IAEA investigators. Though the earlier talks had not made any progress, both sides were optimistic on clinching a deal soon. After the Dec. 13 meeting with Iran, Herman Nackaerts, deputy director general of the IAEA, said that a deal might be finalized by the teams at their meeting in January.

The failure to arrive at an agreement was considered a setback to the efforts to stop Iran’s controversial nuclear program and the attempts to thwart the possibility of a war in the Middle East, the sources said.

Both the teams agreed to meet Feb. 12.

The IAEA suspects Iran of clandestinely developing the nuclear weapons while the latter vehemently denies the charge. Iran claims that its nuclear program is for peaceful purposes. However, the Islamic nation refuses to give the inspectors from the IAEA an unconditional access to sites like Parchin military base, where Iran is suspected to have conducted nuclear-based explosive tests.

Citing satellite images, the IAEA has reported increased activities, including soil replacement, at Parchin in recent months. Tehran claims that Parchin is a conventional military site and denied allegations of cleaning up the area.

The U.S. and Israel have threatened to attack Iran if it fails to cooperate with the nuclear watchdog. However, Iran, which is facing tough sanctions from the West and is under international pressure to cooperate with the IAEA investigations, has been adamant.