Amy Searight, US deputy assistant secretary of defense for South and Southeast Asia, said Washington welcomed EU calls for a peaceful resolution of territorial disputes in the sea and respect for international law.
But there was "somewhat of a difference of approach" when it came to Washington's call for a freeze on activity by rival claimants - something China has rejected.
"It would be helpful if the EU would be a little more clear in terms of backing up these principles," she told a discussion on U.S. and EU policies towards East Asia at Washington's Center for Strategic and International Studies.
"A little bit more forward-leaning approach that would support, for example, the idea of a halt to further reclamation, further militarization, would be very useful."
Michael Fuchs, US deputy assistant secretary of state for East Asia, said there was a need to reduce the risks of conflict in the East Vietnam Sea, where China has overlapping claims with several states.
"This is about ... speaking up when we see activity that is concerning," he said.
David O'Sullivan, EU ambassador to Washington, said the European Union and the United States had very similar objectives, but such statements were a judgment call.
"Completely joining up language is sometimes useful and sometimes counter-productive," he said.
O'Sullivan said the European Union was concerned about security in East Asia and was adding a security dimension to its work, but made clear there were limits to this.
"The last thing the region needs is more gunboats. I don't think that's going to be our contribution to the future security of the region."
Japan's navy chief, Admiral Tomohisa Takei, told another Washington think tank that Asian countries needed to improve their naval capabilities and increase coordination given the East Vietnam Sea tensions.
He said they should improve relations with Washington through "a solid alliance like the Japan-U.S. alliance or friendly relations with the U.S."
Japan, which is not a claimant in the East Vietnam Sea but has huge interests in keeping trade routes open and is in dispute with China over territory further to the north, could help to improve regional naval capabilities, he said.
"I believe that Japan will make both personnel and material contribution toward capacity building."
Japan eased an arms export ban last year and has since agreed to bolster security ties with several countries, including the Philippines, Vietnam, Malaysia and Indonesia, that share its concerns about China.
