In a telephone call, US Secretary of State John Kerry told Chinese Foreign Minister Wang Yi the US had "strong concerns" over recent developments.
Last week, Chinese and Vietnamese ships clashed over the drilling rig that China has placed in waters near the disputed Paracel Islands.
China and Manila are also at odds over overlapping territorial claims.
Beijing claims a U-shaped swathe of the South China Sea that covers areas other South East Asian nations say are their territory.
The issue has been rumbling in recent years amid an increasingly assertive stance from China over its claims.
Last week, several collisions were reported between Chinese and Vietnamese vessels in disputed waters as the Vietnamese ships sought to prevent the installation of the Chinese drilling rig.
Reports suggest the stand-off - involving dozens of ships - is continuing, with water cannon fire being exchanged.
On Sunday, anti-China protests took place in several Vietnamese cities. It is the most serious confrontation between the two nations for several years.
In Monday's telephone call, Mr Kerry "said China's introduction of an oil rig and numerous government vessels in waters disputed with Vietnam was provocative", US State Department spokeswoman Jen Psaki said.
"He urged both sides to de-escalate tensions, ensure safe conduct by their vessels at sea, and resolve the dispute through peaceful means in accordance with international law."
Separately, in comments made at a meeting with the Singaporean foreign minister, Mr Kerry called the Chinese move an "aggressive act".
Mr Wang, meanwhile, urged Mr Kerry to be objective on the issue, a Chinese foreign ministry spokeswoman said.
Tensions between Beijing and Manila are also running high, after the Philippines arrested and then charged nine Chinese fishermen with poaching at a disputed shoal.
The Philippines is in the process of taking China to a UN court over its territorial claims.
It has also recently signed a new security deal with the US which would allow an expanded US troop presence, in a move seen as linked to Manila's deteriorating ties with Beijing.
On Tuesday, Chinese state media slammed the Philippines, saying that if it continued to challenge China it would pay "an unaffordable price".

Wayne Madsen (70) batted well before the innings stalled in the twilight, as the pink ball swung more.
Marchant de Lange (3-82) and Timm van der Gugten (3-88) were particularly dangerous while Graham Wagg weighed in with two wickets.
But Derbyshire flourished in the last session through Daryn Smit (41) and Tom Milnes (53).
Much of the day's action was played under thick cloud cover and wicket-keeper Chris Cooke snaffled five catches as the seamers found plenty of movement.
Meanwhile Derbyshire's 16 year-old Afghan-born spinner Hamidullah Qadri, who made 11 not out, is the first Championship player to have been born in the new Millennium.
Glamorgan coach Robert Croft told BBC Wales Sport: "It's different, certainly I've very open-minded to see how it pans out. After 40 overs, it seemed to get some swing, but I put it down to good bowling rather than conditions.
"If the ECB and world cricket are serious about making pink-ball, day-night four-day and Test match cricket a thing of the future, we have to play more of it to get people used to it,"
Wicket-keeper Chris Cooke added: "The pink ball played pretty well and we'll see how good that score is, but it showed there was a bit in it for both bat and ball under lights.
"Marchant bowled excellently in that (second session) spell and Waggy with him when the lights came on, but Tom Milnes showed you could definitely bat under lights. It's new for everyone so it's whoever adapts quickest might have the edge."
Derbyshire batsman Wayne Madsen told BBC Radio Derby: "It's different having to change routines but we adapted to it quite well, it's a pretty good score we've put on the board considering the conditions, particularly in that middle session where we lost a few wickets and they bowled really well.
"I felt really good at the crease, they've got a decent four-pronged attack and it was a good challenge but conditions when it got overcast made it a lot harder to bat and our guys who came in (later) did a fantastic job."
The US has warned China that moving a drilling rig into seas disputed with Vietnam was "provocative".
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Derbyshire recovered well to 288 all out against Glamorgan, who reached 5-0 at the close in the day-night game.