A Sri Lankan delegation will be in Washington this week to press ahead with further negotiations on US President Donald Trump's tariff on the island nation before the August 1 deadline ends, Foreign Minister Vijitha Herath said.

"They will leave on July 18 to try and work out a bilateral agreement," Herath told reporters on Tuesday.

He said Sri Lanka would try its best to win a further reduction in tariff from the 30 per cent indicated in the letter received from the Trump administration last week.