The US threat of sanctions will not delay Russia's delivery of S-400 air defense systems to India and the two sides have worked out unconventional arrangements for payments, Russian deputy foreign minister Sergey Ryabkov said on Wednesday.

Russia believes India's role in the development of Afghanistan is "indispensable", Ryabkov told a small group of Indian reporters, days after US President Donald Trump mocked India's development aid for the war-torn country.

Moscow has said it won't be pressured by US sanctions while finalizing big-ticket armed hardware deals with New Delhi. Russian officials have said the first release under the $5.4-billion deal for the S-400 systems will be made in 2020.

"You may rest certain our commitments and obligations under the contract would be fully honored and India would receive this system as decided upon in due time and without any delays. Thus, the national security of the country would be improved considerably," Ryabkov said, without setting a time frame for deliveries.

Indian and Russian experts production with military deals have "almost all necessary tools", including substitute ways of payments, he said, adding it will be counter-productive to publicly discuss schedules for payments.

"The political message is important though – we want to be immune to the arbitrary and independent procedures that are taken by some countries against very justifiable forms of international cooperation," said Ryabkov.

Russia, he said, understands India's "essential role in Afghanistan" and welcomes the country's contribution in all efforts aimed at understanding and peaceful resolution of the situation. "We are very much in sync with India on this very important issue," he said.

Development and sustainability of institution are key in countries affected by war and the "Indian effort in material support to Afghanistan is an indispensable one and of utmost importance", he added.

Ryabkov also said it is necessary to have a dialogue with the Taliban "to understand better where they are". He added: "This conflict should be concluded and the only way out is a political one."