Prime Minister Narendra Modi is making a two-day visit to Vietnam to strengthen defence and security ties against the backdrop of China's expanding presence in Asia.

Today, he is scheduled to meet Vietnamese leaders, including Prime Minister Nguyen Xuan Phuc and Communist Party of Vietnam general secretary Nguyen Phu Trong.

Interestingly, Mr Modi's stop in Vietnam is on his way to the Group of 20 summit in China. Officials said that the visit would allow him to engage with Vietnam's leadership.

"My government attaches a high priority to our bilateral relations with Vietnam," said Mr Modi in a statement ahead of his departure for Vietnam. "The India-Vietnam partnership will benefit Asia and the rest of the world."

During Mr Modi's visit, the two countries are likely to sign an agreement to transfer four patrol boats to Vietnam, which has also been keen to source BrahMos cruise missiles from India as it seeks to build up its military capabilities.

The trip comes at a time when the South China Sea is dominating the discourse. China claims most of the South China Sea, putting it in direct conflict with Vietnam and a host of other countries. While India is not directly involved in that dispute, it is concerned about China's growing presence in its neighbourhood. China has built ports and is involved in multiple road projects in various countries, from Nepal to Sri Lanka.

Vietnam, on the other hand, sees India as a counter to China's assertiveness. It has been keen on India undertaking oil exploration in blocks off its coast, in spite of Chinese protests that some parts were in disputed waters. India is currently exploring an oil block in the South China Sea.

Analysts said Mr Modi's visit was about boosting strategic ties in general. "What they (India and Vietnam) want is more than bilateral trade. It is simply to work strategically. Vietnam looks to India to provide a strategic option vis a vis the South China Sea," said retired Indian Navy vice-admiral Pradeep Chauhan. "The most feisty neighbour of China is Vietnam. If India is stitching together a pretty strong relationship with Vietnam, this offers an alternative to the Chinese approach. This constructive engagement (on India's part) is apparent in the Indian Ocean and increasingly in the Pacific."

Since becoming Prime Minister in 2014, Mr Modi has made a conscious effort to further ramp up engagement with Vietnam, with which India shares traditional ties.

Over the last two years, President Pranab Mukherjee and external affairs minister Sushma Swaraj have also visited Vietnam, and India extended a US$100 million (S$136 million) export credit to Vietnam for defence deals in 2014. Bilateral trade between India and Vietnam touches US$8 billion.

The countries' defence relationship dates back to the 1990s. More recently India has even provided submarine training for Vietnamese sailors.

"Our relations with Vietnam are characterised by warmth and a high degree of trust," said Ms Preeti Saran, secretary in the Ministry of External Affairs.

"Vietnam is the central pillar of India's Act East policy. Our priority is a whole host of areas, from defence and security, trade and commerce to maritime cooperation."