WASHINGTON (Reuters) - Republican presidential candidate Mitt Romney blamed President Barack Obama for a "lack of leadership" on the crisis in Syria on Thursday and accused him of abdicating the U.S. role to the United Nations.

Romney turned the U.N. Security Council's failure to approve a Western-backed resolution threatening sanctions against the Syrian regime into an assault on Obama's foreign policy. Russia and China resisted U.S. pressure and vetoed the resolution.

"While Russia and Iran have rushed to support (Syrian President) Bashar al-Assad and thousands have been slaughtered, President Obama has abdicated leadership and subcontracted U.S. policy to Kofi Annan and the United Nations," Romney said in a statement.

(Reporting by Steve Holland; Editing by Jackie Frank)