The United States is guilty of maintaining "double standards" on terrorism because of its practices regarding the Syria conflict, Iran wrote in a letter to the United Nations Wednesday.

A copy of the letter was obtained from Iran's mission to the United Nations by Xinhua News Agency.

The letter was from Iran's Foreign Minister Ali Akba Salehi to UN Secretary-General Ban Ki-moon. It welcomed Ban's denunciation of the Feb. 21 car bombings in Damascus that killed more than 50 people and injured hundreds.

Salehi expressed disappointment the Security Council failed to issue a similar statement on the twin suicide attacks.

A spokesperson for Ban could not immediately confirm receipt of the letter.

"While thanking you for condemning this terrorist and inhumane operation, it is expected from the United Nations Security Council to do the same," the letter said.

"The United States opposition in preventing the UN Security Council to condemn these crimes is indicative of the selective and double standard policy of this country in the fight against terrorism," it said. "Without any doubt this approach would encourage the terrorists to commit similar crimes."

"The Islamic Republic of Iran believes that the continuation of violence and blind explosions only delays the political solution to the conflict and it is a sign of desperation and frustration of the quarters whose interests seems to be better served in the instability of Syria," the letter said.

Published reports said another permanent member of the Security Council, Russia, had complained after closed-door consultations shortly after the bombings of the panel that the United States had blocked its proposal for a statement expressing condolences to Syria for the attacks and to condemn terrorism.

The U.S. and its allies wanted to add a denunciation of Syrian President Bashar al-Assad to the statement, something Russia could not accept.

There was no immediate reaction from the U.S. Mission to the United Nations regarding the Salehi letter.