The group counted nearly 50 community evictions last year alone, affecting almost 12,000 families.
The measures are designed to crack down on the sales of pirated U.S. products overseas.
The letter was delivered to China, France, Russia, the UK and the United States.
Phan Imex was supposed to build 10 buildings to house evicted residents and built eight instead.
A US official said this week that Obama could participate in an Asean summit to be hosted in Cambodia in November.
Asean countries are facing a multitude of issues which includes a dispute over the South China Sea.
Meanwhile, the US is making diplomatic efforts to improve its relationship in Cambodia.
Thirty-eight people remain in unlawful detention after they were seized by authorities as they protested outside City Hall this week.
The demonstrators were seized Wednesday, where they had demanded the courts release eight representatives arrested earlier this month.
Around 300 families lost their homes in the Jan. 3 forced eviction of Borei Keila.
An estimated 20,000 Cambodians are trafficked throughout the region each year.
The site, at Phnom Bath mountain, is 50 kilometers outside the capital and lacks clean water, electricity, and health care facilities.