The White House on Wednesday sounded an optimistic note after this week's first round of nuclear talks with Iran, but cautioned against expecting an immediate “breakthrough.”

“We found the Iranian presentation very useful. ... [It] was a new proposal with a level of seriousness and substance that we had not seen before,” White House spokesman Jay Carney told reporters.

“Having said that, no one expects a breakthrough overnight,” he added, noting that the mistrust on both sides is “very deep.”

Two-day talks in Geneva between a group of six world powers and Iran ended Wednesday after Iranian officials pledged to make some concessions on enrichment levels and promised to allow international inspectors to monitor Iran's nuclear sites.

U.S. lawmakers had previously warned the Obama administration to ensure Iran provides verifiable commitments after new President Hassan Rouhani said Tehran would reserve its right to enrich uranium during a speech to the United Nations earlier this month.

Despite the upbeat assessment of initial talks, the White House said Iran must still prove its commitment to comply with international treaties and U.N. Security Council resolutions.

“The onus remains on Iran to come into compliance with its international obligations, and any deal must prove to the international community that Iran's program will be used for exclusively peaceful purposes,” Carney said.

The European Union's High Representative for Foreign Affairs Catherine Ashton said the P5+1 group and Iran will meet again in Geneva on Nov. 7th and 8th. The group, which includes the United States, Britain, France, Russia, China and Germany, issued a joint statement with Iran saying the negotiations were “substantive and forward looking.”

Ahead of this week's talks, a bipartisan group of lawmakers pressed President Obama to stay firm on Iran sanctions and not make concessions until Tehran takes tangible steps to eliminate its nuclear weapons program.

The lawmakers said Tehran needs to comply with the International Atomic Energy Agency, the Nuclear Nonproliferation Treaty and all U.N. Security Council resolutions, which would require an immediate halt to enrichment.

The House overwhelmingly approved stricter sanctions on Iran in July, but the Senate Banking Committee followed the Obama administration's advice to hold off on new sanctions to give Tehran time to prove its commitment to slowing its nuclear program and providing more transparency for international inspectors.

Western nations point to Iran's enrichment program as evidence that it is trying to develop weapons, while Tehran insists it seeks nuclear energy for peaceful economic purposes.