HANOI • Vietnam must take steps to cut its trade surplus with the United States, US Trade Representative Robert Lighthizer has said, as the Trump administration ramps up pressure on the Asian country.

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

Measures which 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 reached US$40 billion (S$55 billion) last year, the highest in records going back to 1990, according to US Census Bureau data. For the first five months of this year, the surplus is already 43 per cent higher than a year ago, at US$21.6 billion.

US President Donald Trump described Vietnam as "almost the single-worst abuser of everybody" when asked last month if he wanted to impose tariffs on Hanoi.

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

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

BLOOMBERG