The White House on Wednesday would not characterize the situation in Syria as a civil war, but warned of such a scenario as the window of opportunity is closing.

White House spokesman Jay Carney would not say directly whether Syria is in the midst of a civil war, but said "What the administration believes is that the situation there is deteriorating, it is deteriorating quickly."

"It is horrific what Assad is doing to his own people, and that the window of opportunity to bring about a transition to a democratic future for Syria is closing and will close, and if it does, the chance for a broader sectarian civil war in Syria will be enhanced greatly," he told reporters at a regular news briefing.

Syria's Foreign Ministry on Wednesday rejected UN official' statement that the country is in a state of civil war, saying the country does not witness a civil war but rather a struggle to eradicate terrorism and encounter killings.

A day earlier, Herve Ladsous, UN under-secretary for peacekeeping operations, said Syria is in a state of civil war and the government has lost control of "large chunks" of cities.

Washington and its Western allies have long urged Syrian President Bashar al-Assad to step down to make way for a political transition in the country. They have been using a recent spike in violence to condemn the Syrian government and have stepped up pressure on Damascus.

Russia and China, both permanent members of the UN Security Council, are advocating a more balanced approach to the Syrian crisis and have urged all relevant Syrian parties to quit violence to pave the way for a negotiated resolution.

Carney warned that if countries do not act quickly to bring about a transition in Syria, "we will be in a situation, or we will likely be in a situation where Syria is experiencing a sectarian civil war that could spill beyond its borders, that could destabilize the region, that could involve other countries in the kind of proxy conflict that can be very damaging not just to the region but to the world."

Syria's Foreign Ministry on Tuesday accused Washington of encouraging armed terrorist groups to "carry out more massacres" in the country and of "flagrant interference in Syria's internal affairs."