WASHINGTON: The United States showed heightened concern over a Syrian offensive in second city Aleppo on Friday, but rejected comparisons to a Libyan crackdown that triggered international intervention.

"We are very concerned about the situation in Aleppo," White House spokesman Jay Carney said, condemning President Bashar Assad's "heinous, reprehensible" assault on civilians.

"The kinds of weaponry that they're using against unarmed civilians I think demonstrates the depths of depravity to which Assad has sunk," he told reporters.

Carney was asked about the similarities between Aleppo, a restive stronghold for the rebel Free Syrian Army, and Benghazi, the rebel-held Libyan city that was an early focus of the uprising against Libyan leader Moammar Gadhafi.

Gadhafi's threat to crush the uprising in Benghazi was cited by an international coalition that included the United States to intervene militarily in Libya last year to enforce a no-fly zone.

Carney pointed to a "broader array of issues" that led the United States and its allies to launch their offensive.

"There was the imminent assault. There was the call from the opposition, the unified opposition, for international action," he said.

"There was international consensus both at the level of the United Nations Security Council as well as regional consensus through the Arab League."

In Syria, however, "we do not have that," Carney said, reiterating U.S. "disappointment" with Russia and China's decision to veto three U.N. Security Council resolutions on Syria.

But a group of three U.S. senators did not hesitate to compare the Aleppo crisis to Benghazi.

Republican Senators John McCain and Lindsey Graham, along with their Independent colleague Joe Lieberman, recalled that the NATO-led offensive averted a "massacre" and helped the Libyan people "win their freedom and liberate their country" from Gadhafi, who was killed.

"It is not too late for the United States to make the difference in Syria, as we did in Libya," they said in a statement.

"We can and should be providing weapons, intelligence, and training directly to the rebels -- not sitting on the sidelines and outsourcing this job to others."

The senators condemned Washington for "failing to take any of the steps that are within our power to stop Bashar al Assad's killing machine."

Syrian gunships pounded rebels and troops clashed with insurgents in Aleppo earlier amid global fears of an all-out assault.

Columns of armed forces and armor have poured into Aleppo over the past two days, with troops firing on a string of rebel neighborhoods in the battle for control of the pivotal northern city.