US Senator John Kerry (R) with US Secretary Hillary Clinton on Capitol Hill in Washington
US Senator John Kerry has said America will do what it must to stop a nuclear Iran, at a friendly hearing for his confirmation as secretary of state.
President Barack Obama nominated Sen Kerry, 69, to replace Hillary Clinton at the helm of the state department.
The Massachusetts Democrat is expected to win easy confirmation, reports the BBC.
Sen Kerry was Obama's second choice, after UN Ambassador Susan Rice became embroiled in controversy over a raid on a US consulate in Benghazi, Libya.
A Vietnam veteran and a senator since 1985, Sen Kerry was the Democratic presidential nominee in 2004, losing the election to George W Bush.
He is chairman of the Senate foreign relations committee, which is conducting Thursday's hearing. The session is led by Senator Robert Menendez of New Jersey.
Sen Kerry said the US was "totally" committed to enforcing US sanctions against Iran.
"The president has made it definitive," he said, "we will do what we must to prevent Iran from obtaining a nuclear weapon.
"I repeat here today: our policy is not containment. It is prevention and the clock is ticking on our efforts to secure responsible compliance."
Sen Kerry was introduced by current Secretary of State Hillary Clinton, Republican Senator John McCain and new Massachusetts Senator Elizabeth Warren.
"John is the right choice," Mrs Clinton told the panel, as she warmly praised her designated successor. "He will bring a record of leadership and service that is exemplary."
Her recommendation was immediately echoed by Sen McCain.
"Witnessing almost daily his exemplary statesmanship is one of the highest privileges I've had here," he said.
After service as a naval officer in Vietnam, during which he was wounded on a riverine combat patrol, Sen Kerry graduated from law school at Boston College.
He served as a prosecuting attorney before entering politics. After a stint as lieutenant governor of Massachusetts he was elected to the Senate in 1984.
Sen Kerry's expected ascent to secretary of state opens his Senate seat. The seat will temporarily be filled by appointment by Democratic Massachusetts Governor Deval Patrick; a special election will follow within months.
Ms Rice's nomination became bogged down amid fierce opposition from Sen McCain and other Republican senators.
They accused her of misleading the American people over the nature of an attack on 11 September last year against the US consulate in Benghazi, Libya, in which four Americans were killed.
Days after the raid, Ms Rice said on a chat show that the attack had grown out of an anti-American protest outside the consulate.
Officials have since said her account was inaccurate, that the raid was a planned attack by militants, but Ms Rice denied being intentionally misleading.