A long-awaited trade agreement between India and the United States is almost complete, with only one or two per cent of negotiations remaining, a senior US official said, signalling that the two countries are approaching a breakthrough after months of high-level discussions.

The official said the biggest issues had already been resolved and that negotiations were continuing at the highest political levels, including between President Donald Trump and Prime Minister Narendra Modi.

"The biggest items have been settled. I mean …., it's pretty much done. We have that one, one or two per cent left," the official told reporters during a State Department briefing on Friday.

The remarks came ahead of Secretary of State Marco Rubio's scheduled visit to India, where he will hold discussions with Indian government officials and leading business executives in Mumbai and Jamnagar.

The official said the two governments remained in constant contact over the proposed agreement.

"We are actively engaged every single week," the official said, adding that high-level discussions on the trade deal were taking place regularly.

The official cited recent diplomatic engagements involving US Trade Representative Jamieson Greer, India's Commerce Minister Piyush Goyal and conversations between Trump and PM Modi as evidence of the momentum behind the negotiations.

However, the official also identified a significant complication that had delayed the agreement.

An earlier understanding had been linked to the International Emergency Economic Powers Act, or IEEPA, but the legal framework had to be reconsidered after the law was struck down, according to the official.

The development forced negotiators to revisit elements of the proposed agreement and contributed to several additional months of discussions.

Despite those complications, the official expressed optimism that the remaining differences could be resolved soon.

Rubio is scheduled to travel first to Uzbekistan for meetings with Central Asian leaders before proceeding to India.

In Mumbai, he will meet Indian government counterparts and senior executives from some of the country's largest companies.

The official said the decision to visit Mumbai rather than New Delhi reflected the administration's growing emphasis on commercial diplomacy and Indian investment in the United States.

Rubio will also travel to Jamnagar.

The official said Washington was seeking closer engagement with India across information technology, pharmaceuticals, defence cooperation and other sectors.

Further visits by senior American Cabinet officials are also anticipated next year as both governments seek to expand bilateral engagement.