The deal is likely to happen within this week on October 4-5, when Russian President Vladimir Putin will visit New Delhi to attend the annual India-Russia summit. It is also said that the sensitive radars of the missile system can detect even stealth aircraft which are impossible for other systems to detect.
Sources said Modi and Putin will explore ways to further deepen cooperation between the two countries in a number of sectors including trade, investment, connectivity, energy, space and tourism.
However, it will be the first official meeting between India and Russian Federation on the signing of the deal.
Mr Putin also admitted that Moscow knew that Mr Skripal had cooperated with foreign intelligence upon his release in a 2010 spy swap. Tensions have been further exacerbated by disputes over Ukraine, Syria and Iran as well as USA allegations of a Russian military threat to Europe.
The S-400 system, which is effective against aircraft as well as ballistic and cruise missiles, has a maximum range of 250 miles. India will be purchasing five S-400 Triumf missile air defence systems.
Modi's government, caught in the U.S. -Russia crossfire, hopes the Trump administration will give it a pass on the proposed arms transfer from Russian Federation, officials in New Delhi said.
"The key feature of this visit will be the signing of the agreement to deliver S-400 air defence systems", Putin's top foreign policy aide Yury Ushakov said, according to news agency AFP. "We have a big legacy of buying defence equipment from Russian Federation".
OPEC and its non-cartel partners - including Russian Federation - agreed at the end of June to increase production after a previous agreement at the end of 2016 had limited supply to support prices.
The Modi government hopes the Trump administration will not disrupt burgeoning defence ties with the world's biggest arm importer by levying sanctions the U.S. Congress has sought.
"The administration was critical of the CAATSA and fought hard for a national security waiver at the cabinet secretary level but was denied by Congress", he said.