WASHINGTON (AP) — Secretary of State Marco Rubio said Tuesday that he is optimistic about the potential for a resumption in nuclear talks with Iran despite a shaky ceasefire in the war that is looking increasingly in doubt.

Rubio testified before a House Appropriations subcommittee at 2 p.m. EDT. Watch in our video player above.

Rubio defended the Trump administration's approach to Iran and other global hotspots in back-to-back hearings before the Senate Foreign Relations Committee and a House Appropriations subcommittee. He was briefly disrupted by protesters at each session.

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In his first public testimony since the Iran war began at the end of February, Rubio said the Iranians have agreed to negotiate on nuclear points that they had not been willing to address in the past but would not offer an assessment on what those talks might produce.

Rubio testified before the Senate Foreign Relations Committee at 10 a.m. EDT. Watch the full hearing below.

"They have agreed to negotiate aspects of their nuclear program that just a month ago, just a year ago, they were refusing to even mention," Rubio told the Senate. He noted, however, that there was no guarantee "it will lead to a deal that's acceptable" and that negotiations have been made difficult by the instability of Iran's leadership.

Rubio's optimism ran counter to pessimistic reports from two semiofficial Iranian news agencies that Iran has stopped communicating with mediators after Israel threatened to bomb Beirut as it fights the Hezbollah militant group. President Donald Trump disputed that Iran has cut off communication with mediators, calling Iranian reports of a cessation in talks "false and erroneous."

Democrats criticize Trump administration's approach to Iran, and Rubio defends it

Rubio's wide-ranging testimony was met with fierce objections from Democrats, including tough questions about the status of U.S. foreign assistance to respond to diseases such as the Ebola outbreak in Africa. Rubio insisted that the dismantlement of the U.S. Agency for International Development had not affected Washington's ability to assist with global humanitarian responses.

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Sen. Chris Van Hollen, D-Md., blasted Rubio and Trump for foreign aid cuts and overseas intervention. Van Hollen specifically took aim at the U.S. and Israeli decision to strike Iran, accusing the Republican president of entering the war on behalf of Israel.

Israeli Prime Minister Benjamin Netanyahu "said he's been waiting 40 years to do this," Van Hollen said. "It turns out he finally found a president who was both stupid and reckless enough to join him. Let's face it, Mr. Secretary, the Trump foreign policy has become a dumpster fire."

Rubio's testimony, which was taking place as Israel and Lebanon began a new round of political talks at the State Department with the situation between Israel and Hezbollah still uncertain, did not provide definitive answers on any of the main questions of the day.

He said Iran is not guaranteed a massive payout for reopening the Strait of Hormuz, the crucial waterway for global oil shipments, and would have to commit to further concessions on its nuclear program to get significant sanctions relief.

"The more they give, the more they would get," he said, later adding, "They're not going to get it as a signing bonus."

Rubio also said there are indications that Iran's new supreme leader, Mojtaba Khamenei, is taking a bigger part in the discussions despite not being seen publicly since the war began.

"I think there are indications out there that he is increasingly engaging at some level, although all of his communications have been in writing and through intermediaries," he said.

Democratic senator says drugs being on boats isn't a targeting criterion for U.S. strikes

On other issues, Rubio dismissed questions about the legality of Pentagon strikes against dozens of alleged drug-smuggling vessels in the Caribbean Sea and the eastern Pacific Ocean, which have killed more than 200 people since early September.

Democratic Sen. Tim Kaine of Virginia said the military's targeting criteria for those strikes does not include drugs being present on the boat. He called it "odd" but said he could not share much more because the criteria are classified.

Rubio pushed back, saying every strike has a legal officer who makes a determination on whether a strike is legal. He also said the U.S. military has "walked away from strikes" multiple times because they did not meet the targeting criteria.

The Trump administration says the U.S. is at war with drug cartels, while many Democrats have questioned the legality and effectiveness of the strikes.

The Republican former senator faces a second congressional hearing Tuesday and a pair of others Wednesday about the State Department's annual budget request, though questions have mostly focused on top foreign policy issues.

Rubio wades into Taiwan arms sales opposed by China

Rubio acknowledged that the Trump administration is holding up a new potential $14 billion arms sale to Taiwan but said it remained under consideration and would not be canceled. He noted that the U.S. recently sold arms to Taiwan in December worth $11 billion.

He said the deal is not under review because of pressure from China, although he said the Chinese bring up the issue in discussions with the United States. Trump also has described it as a great negotiating chip.

"They are constantly talking about Taiwan arms sales, but that in no way is what is holding up our decision-making or the White House's decision-making," Rubio said. "It is something the president will have to decide on the timing of when and how that is executed on."

Protesters chant at Rubio about Cuba

Rubio, the son of Cuban immigrants, also was questioned about the Trump administration's escalatory behavior toward Cuba, as Trump has hinted that the small island country could be the next U.S. target after operations in Iran are wrapped up.

He faced chants from protesters who urged him to "stop killing Cubans" when he entered the Senate briefing room. The protesters were quickly pulled from the room. Their chants also included "Let Cuba live!"

Rubio defended the administration's approach to Cuba and said it would remain focused on changing the Cuban government's policies.

"I really don't believe this system is capable of reform unless new people take over or a new mindset takes hold," he said.

Despite a series of meetings between U.S. and Cuban officials, Trump and Rubio have renewed threats against the island's government, which take on greater weight after the administration announced criminal charges against former President Raúl Castro.

Over his congressional career and now as America's top diplomat, Rubio has maintained that Cuba is a national security threat because of its ties to U.S. adversaries and that Trump is intent on addressing it.

Amiri reported from New York.