Lam's visit to Beijing, his first overseas trip since he was appointed party chief early this month, signals a desire between the two communist neighbours to strengthen ties as trade and investment grow and despite occasional clashes over boundaries in the South China Sea.

"China has always regarded Vietnam as a priority in its neighbourhood diplomacy, and supports Vietnam in adhering to the Party leadership, taking the socialist path suited to its national conditions, and deepening the cause of reforms and socialist modernisation," Xi said, underscoring establishing good working relations and a personal friendship with Lam.

Lam described the bilateral ties as a "top priority in Vietnam's external policy" and called his trip to China "the affirmation of the Party and the Vietnamese government to value the relation with China".

The two countries signed documents on planning and feasibility studies for standardised railway routes, in what appears as a new step towards the upgrade of cross-border rail links, after preliminary deals on the matter were signed in December during Xi's state visit to Hanoi.

In December, both countries said they would work on cross-border railway connectivity, naming three projects that included one connecting through mountainous Lao Cai in the Vietnam's northwest to the port city Haiphong and a potential one linking China's Shenzhen to Haiphong.

Vietnamese officials had said rail links would be high on the agenda when the top leaders meet.

The two nations are connected by two railways from southern China to Vietnam's northern industrial hub and capital Hanoi, but the Vietnamese infrastructure dates back to French colonial days and has a different gauge from Chinese high-speed rail, forcing passengers and goods to swap trains at the border.

Upgrading the Vietnamese side of the railways could boost trade and investment, as a growing number of Chinese manufacturers move some export-oriented operations to Vietnam amid trade tensions between China and the United States.

Other documents signed covered cooperation between central banks, media, health and the quarantine and inspection of coconuts, crocodiles and durians.

After the signing, Xi and Lam continued talks on important issues of common concern over tea in a "cordial and friendly atmosphere", Chinese official media Xinhua said.

The two countries will issue a joint declaration on further strengthening their strategic partnership, Xinhua said.

Lam arrived in China's southern province Guangzhou on Sunday for a three-day visit that would include meetings with Chinese Premier Li Qiang and other Chinese top officials.

While in Guangzhou, he visited some Chinese locations where former President Ho Chi Minh conducted revolutionary activities.

China and Vietnam forged diplomatic ties in 1950 and established a comprehensive strategic partnership of cooperation in 2008 that was jointly fortified five years later to extend to more shared international and regional issues of concern.