U.S. Energy Secretary Ernest Moniz, U.S. Secretary of State John Kerry, Iran's Foreign Minister Mohammad Javad Zarif and the head of the Atomic Energy Organization of Iran Ali Akbar Salehi, from left to right, pose for a photograph before resuming talks over Iran's nuclear program in Lausanne, Switzerland, Monday, March 16, 2015. Brian Snyder/AP

WASHINGTON (Reuters) - Iran gained access to about $100 billion in frozen assets when an international nuclear agreement was implemented last month, but $50 billion of it already was tied up because of debts and other commitments, a U.S. official said on Thursday.

Stephen Mull, the State Department's lead coordinator for implementing the international nuclear agreement with Tehran, also told the House Foreign Affairs Committee there was no evidence Iran had cheated in the first few weeks since the deal was implemented.

Mull and John Smith, acting director of the Treasury Department office that oversees sanctions, faced heated questioning from some members of the committee, where several Democrats had joined Republican lawmakers in opposing the nuclear pact that was reached in July.

Many have worried that Iran would cheat on the deal and use unfrozen funds for action against Israel or to support Islamist militants elsewhere in the region.

"Of that amount, a significant portion of it, more than $50 billion, is already tied up," Mull said.

It was the first top-level congressional hearing on the nuclear pact since Jan. 16, when world powers lifted crippling sanctions against Iran in return for it compliance with the agreement to curb its nuclear ambitions.

"We seem to be in many instances talking tough about Iran," said U.S. Representative Eliot Engel, the panel's top Democrat, a deal opponent. "In reality our actions are far away from our rhetoric and that's a worrisome thing. We want to make sure that Iran's feet are held to the fire."

Many members of the U.S. Congress, where every Republican and a few dozen Democrats opposed the agreement, have been calling for legislation to impose new sanctions on Iran over its ballistic missile program and human rights record.

House Republicans have been pushing legislation to restrict the ability of President Barack Obama, a Democrat, to lift sanctions under the nuclear pact. One measure passed the House on Feb. 2 almost entirely along party lines but it has not yet been taken up in the Senate and Obama has promised a veto.