The White House has accused Iran of "malignant behaviour" in propping up Syrian President Bashar al-Assad.
It warned anew that the country's conflict could explode into a wider proxy war unless Mr Assad steps down.
White House press secretary Jay Carney said that Iran was exploiting the violence in Syria to entrench its regional sway.
"That fact further highlights Iran's continued effort to expand its nefarious influence in the region, and underscores Iran's fear of a Syria without the Assad regime," he said.
European and US security officials say Iran has offered Mr Assad extensive support, including weapons and ammunition, to shore up a vital ally.
Mr Carney's comments came amid growing concern in some world capitals that the Syrian bloodshed could devolve further into a proxy war - with Iran being only one of the outside players.
The Gulf state of Qatar, a close ally of Washington, has provided weapons to the Syrian opposition, according to Western officials, and there have been hints Saudi Arabia has done likewise.
Some US politicians want President Barack Obama to arm the opposition, with pressure hardening after a weekend massacre blamed on Mr Assad.
Human rights groups and Western officials said today that a Russian cargo ship heavily laden with weapons docked at the Syrian port of Tartus last weekend.
Susan Rice, US ambassador to the United Nations, yesterday laid out a worst-case scenario in which Syria could become a proxy conflict, prompting world powers to take unilateral action.
Mr Carney, claiming that Iran was engaged in "malignant behavior" in Syria, said the international community must up pressure for Assad to leave to stop the conflict from widening.
He said Washington was working with the Russians, who together with China have previously vetoed tougher UN Security Council action against Mr Assad, to persuade them of the "horrific" risks of allowing the conflict to escalate.
Russian President Vladimir Putin has given no sign that he will drop Moscow's opposition to tougher sanctions or related action against Syria.
Washington is separately pushing, along with European allies, for Tehran to curb its nuclear program, which the West says is secretly pursuing an atomic bomb but Iran insists is purely for peaceful purposes.
Meanwhile, the Syrian government has blamed up to 800 rebel fighters for the massacre in central Syria last week that killed more than 100 people, nearly half of them children, in its most comprehensive explanation to date of the bloodshed.
The narrative starkly contradicted accounts of witnesses who blamed President Bashar Assad's regime.
The UN also said it had strong suspicions pro-regime gunmen were responsible for much of the carnage on Friday in a cluster of villages known as Houla.
Facing international outrage over the killings, Damascus launched its own investigation into the deaths and announced that special prayers for the victims would be held at mosques across the country tomorrow.
Reports from Syria cannot be independently verified as the government has banned foreign media.
