The crews from the Vietnamese and Chinese coast guards shook hands and took photos as they met last month, sharing platters of fruit and raising their glasses for a toast. Now, they are in a tense standoff in the South China Sea.

“The two sides were very happy and united,” Lt. Colonel Phan Duy Cuong, the operations assistant of Vietnam’s Coast Guard Command, said of the April 15 ceremony. “We toasted each other with wine. They went on our boat and we went on theirs.”

Ships No. 8003 and 2007 sailed alongside two Chinese coast guard vessels for three days in the Gulf of Tonkin. A month later, at least one of those Chinese boats has been spotted helping guard an oil rig that Vietnam is demanding be removed from contested waters about 140 miles (225 kilometers) off its coast, according to Cuong.

The dispute over the rig near the Paracel Islands reflects a renewed chill between the two Communist nations after efforts to draw them closer together, including a flurry of official visits last year. Both China and the U.S. have targeted Vietnam as a potential partner to bolster their influence in the region.

“We were working together just days before, but now there is a line dividing us,” Cuong said on board boat 8003, which carried a crew of 50 plus 100 live chickens in a pen on the stern. “I’m very sad.” While on the joint patrol in April, the boats together inspected Chinese and Vietnamese fishing boats.

Read more: http://www.businessweek.com/news/2014-05-18/vietnam-china-shared-patrols-turn-to-south-china-sea-standoff