WASHINGTON: A top Pentagon official has cautioned India that there are no guarantees for a special waiver from US sanctions if it buys new weapons platforms from Russia.

Washington is alarmed at the prospect of India, an increasingly important US military ally and the world's top defence importer, buying new systems from Russia, 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.

But after lobbying from Defence Secretary Jim Mattis, Congress granted the US president and the secretary of state the option of granting waivers, such as when an ally nation transitions from legacy Russian gear to Western arms.

Randall Schriver, the Pentagon's assistant secretary of defence for Asian and Pacific Security Affairs, said the waiver authority had created the impression Washington would insulate India "from any fallout from this legislation no matter what they do." "I would say that's a bit misleading," Schriver told an audience at the Carnegie Endowment For International Peace in Washington yesterday.

"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 was speaking ahead of a historic summit in New Delhi next week, a first ever "2+2" between India and the United States that will see Mattis and Secretary of State Mike Pompeo meet with their Indian counterparts.