"We are strongly concerned about dangerous conduct and intimidation by vessels in the disputed area," she told a regular news briefing. "We call on all parties to conduct themselves in a safe and appropriate manner, exercise restraint, and address competing sovereignty claims peacefully, diplomatically, and in accordance with international law."
Read MoreTensions mount between Vietnam and China in South China Sea
Psaki said the United States had seen reports that Philippine police had seized Chinese and Philippine fishing boats carrying illegally harvested sea turtles in the South China Sea and detained their crews.
"We urge both sides to work together diplomatically," she said. "Given the United States works with the international community to combat wildlife trafficking, we are concerned that the vessels appear to be engaged in direct harvest of endangered species of sea turtles."
China has demanded that the Philippines release its boat and fishermen.
Vietnam's Foreign Ministry said on Wednesday a Chinese vessel intentionally rammed two of its ships in a part of the South China Sea where the giant oil rig was deployed.
It said the collisions took place on Sunday and caused considerable damage to the Vietnamese ships. Six people sustained minor injuries, it said.
The dispute comes days after U.S. President Barack Obama visited Asia to underline his commitment to allies there, including Japan and the Philippines, locked in territorial disputes with China.
China has not yet responded to the Vietnamese allegations of ramming, but Foreign Ministry spokeswoman Hua Chunying said earlier on Wednesday that the deployment of the rig had nothing to do with the United States, or Vietnam.
China claims almost the entire South China Sea, rejecting rival claims from Vietnam, the Philippines, Taiwan, Malaysia and Brunei.