The Chinese government's top diplomat has warned the world should not open a "Pandora's Box" in the Middle East, denouncing US pressure on Iran and calling for the country to remain in its landmark nuclear deal.

Fears of a confrontation between Iran and the US have mounted since last Thursday when two oil tankers were attacked in the Gulf of Oman.

The US blamed Iran for the attacks, more than a year after President Donald Trump withdrew from the 2015 Iran nuclear deal.

Iran denied involvement in the tanker attacks and said on Monday it would soon breach limits on how much enriched uranium it can stockpile under the deal, which had sought to limit its nuclear capabilities.

Acting US Defence Secretary Patrick Shanahan announced on the same day the deployment of about 1000 more troops to the Middle East.

Speaking after meeting with Syria's foreign minister, Chinese State Councillor Wang Yi said the US shouldn't use "extreme pressure" to resolve issues.

"We call on all sides to remain rational and exercise restraint, and not take any escalatory actions that irritate regional tensions, and not open a Pandora's Box," Wang told reporters in Beijing.

"In particular, the US side should alter its extreme pressure methods.

"Any unilateral behaviour has no basis in international law. Not only will it not resolve the problem, it will only create an even greater crisis."

Wang also said the Iran nuclear deal was the only feasible way to resolve its nuclear issue, and urged Iran to be prudent.

"We understand that relevant parties may have different concerns but first of all the comprehensive nuclear deal should be properly implemented," he added.

China and Iran have close energy ties, and China has been angered by US threats against countries and companies that violate US sanctions by importing Iranian oil, including Chinese firms.