The US administration saw an opportunity in Cambodia, when american-friendly Prime Minister Lon Nol executed a coup d'etat against then - Monarch Sihanouk in 1970. Lon Nol promised to get rid of the North Vietnamese troops, who used Cambodia as a retreat, as well as the Khmer Rouge, who were cooperating with the Vietnamese. The US saw this as an opportunity to extend the bombings from Vietnam into Cambodia ('Operation Menu'), and invade the country to hit the Vietcong in the border areas. US military officials even thought of a 'hidden headquarter' of the Vietnamese NLF somewhere in that region. In 1973, those bombings were extended to cover all of Cambodia.

The bombings of the border regions caused Northern Vietnamese troops and their allies to retreat further into cambodian territory, thus further destabilizing the already fragile Situation there, caused by economic crises connected to a drop in rice exports at the end of the 1960s. Cambodia's primary source of income was in fact rice, and when the Vietnam war was in full action, more and more rice was smuggled across the border into Vietnam, thereby reducing Cambodia's revenue.

After the US intervention, which lasted until 1973, Pol Pot and his Communist Party of Campuchea (CPK) used the devastation of US bombings to rile up the population against their other enemies, namely Sihanouk's men, as well as Lon Nol's forces. The CPK propaganda was used to justify purges and to appeal to a feeling of nationalism, in order to recruit more comrades.

I couldn't find anything on American backing of the Pol Pot Regime, but rather that the CPK was mainly funded and supported by the Chinese and later the Northern Vietnamese government. Personally, considering that the Khmer Rouge were some form of very rigid 'stone age' communists, i can't imagine that any US government would have considered supporting them. (The question of how communist they really were is highly interesting, though).

Sources: Frey, Marc: Geschichte des Vietnamkrieges, Munich 2010.

Kiernan, Ben: The Pol Pot Regime. Race, Power and Genocide in Cambodia Under the Khmer Rouge, 1975-79, New Haven/London 2008.

Frieson, Kate: The impact of Revolution on Cambodian peasants, 1970-1975, Monash University, Australia 1991.

Edit: Formatting and minor spelling errors (mobile user)