Reporting by Patricia Zengerle (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 coordinator for implementing the agreement, also told the House of Representatives 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 some Democrats joined Republican lawmakers in opposing the nuclear pact reached in July.

Republican Representative Chris Smith asked whether Tehran ally Russia, which has taken Iran's enriched uranium, could be trusted with it. He asked where the material was, and Mull said he did not know. "That's a Russian government responsibility to decide where it goes," Mull said.

Smith said that was a "flaw" in the agreement. "We don't even know where it is," he said.

Many lawmakers worry 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 such congressional hearing on the nuclear pact since Jan. 16, when world powers lifted crippling sanctions against Iran in return for its 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 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 Democratic President Barack Obama's ability to lift sanctions under the nuclear pact. One measure passed the House on Feb. 2, but the Senate has not taken it up and Obama has promised a veto.

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