The "2+2" meeting in New Delhi of US Secretary of State Mike Pompeo and Defense Secretary Jim Mattis with counterparts Sushma Swaraj and Nirmala Sitharaman is the first of its kind.

It follows Washington's designation in 2016 of India as a "major defence partner" as the US seeks to turn India into a regional bulwark against China.

The talks were first meant to be held in April and then in June but both were postponed, triggering speculation of a rift.

Pompeo said on Tuesday, en route for India's arch rival Pakistan, that India was a "true strategic partner" and "very important to our success in our Indo-Pacific strategy".

"As the world's largest democracy, we see India taking on a greater regional and global responsibility, a stabilising force," Mattis said.

The talks were expected to touch on expanding the scope of joint military exercises.

India and the US are also close to signing an agreement that would ensure communications between their two militaries are compatible and secure.

But Washington also wants India to distance itself from Russia, notably by stepping back from decades of purchases of Russian military equipment.

India, the world's top defence importer, wants to purchase new systems from Moscow, including its S-400 long-range, surface-to-air missile system.

Under current US rules against Russia, third countries could face sanctions if they transact with Russian defence or intelligence sectors.

- Russia deal almost done -

The Press Trust of India recently quoted an unnamed official as saying the deal was almost done, and that Delhi would be asking for a special sanctions waiver from Trump.

However last week, Randall Schriver, the Pentagon's assistant secretary of defence for Asian and Pacific Security Affairs, warned there are no guarantees.

"We would still have very significant concerns if India pursued major new platforms and systems (from Russia). I can't sit here and tell you that they would be exempt, that we would use that waiver," Schriver told a Washington think tank.

India already has bought US Apache attack helicopters and other gear, and is negotiating to buy armed drones.

In May, Trump pulled the US out of the 2015 nuclear deal with Iran and said other countries, including India, must stop buying oil from Tehran before November 4 or face US sanctions.

India however is highly reliant on Iranian crude imports.

Pompeo said on Tuesday that the issues of Russian arms sales and Iranian oil "will certainly come up, but I don't think they will be the primary focus of what it is we are trying to accomplish here."

"There are half-a-dozen things on the agenda that we are really intent on making progress on... They are really about things that are big and strategic and will go on for 20, 40, 50 years."