WASHINGTON (BLOOMBERG) - Vietnam must take steps to cut its trade surplus with the US, Trade Representative Robert Lighthizer said in comments released Monday (July 29), as the Trump administration ramps up pressure on the Southeast Asian country.

The US has a growing trade shortfall with Vietnam, and the government "has been clear with Vietnam that it has to take action to reduce the unsustainable trade deficit," Mr Lighthizer said in written responses to the US Senate Finance Committee.

Measures Vietnam should take include "expanding its imports of goods from the United States and by resolving market access restrictions related to goods, services, agricultural products, and intellectual property," he said.

Vietnam increasingly is being targeted by the Trump administration over a swelling trade surplus with the US, one of its biggest trade markets.

Vietnam's annual trade surplus with the US has exceeded US$20 billion (S$27.39 billion) since 2014 and reached US$40 billion last year, the highest in records going back to 1990, according to US Census Bureau data.

For the first five months of the year, the surplus is already 43 per cent higher than a year ago at US$21.6 billion.

In May, the US Treasury added Vietnam to a watchlist of countries being monitored for possible currency manipulation.

President Donald Trump described Vietnam as "almost the single-worst abuser of everybody" when asked in June if he wanted to impose tariffs on the nation.

Mr Lighthizer also criticised Vietnam for a "host of unfair trade barriers" US businesses face in Vietnam.

Mr Lighthizer, who was answering a question from Senator Mark Warner, testified before the committee June 18. Answers to additional questions from committee members were submitted in writing Monday.

Mr Warner asked if Mr Lighthizer agreed with Mr Trump that Vietnam has taken more advantage of the US than China.

"I'm concerned that the administration is pursuing an ad hoc trade strategy that has the effect of isolating regional partners,"Mr Warner said.