Japanese Prime Minister Yoshihide Suga and Vietnamese Prime Minister Pham Minh Chinh affirmed their opposition to China's growing maritime assertiveness during phone talks Monday, the first between the two leaders since Chinh was elected last month.

In their roughly 30-minute call, the two also voiced grave concerns regarding China's implementation of a new law in February that allows its coast guard to use weapons against ships it views as intruding into its territory.

File photo taken in September 2012 shows the Senkaku Islands in the East China Sea. (Kyodo)

"We want to strengthen ties (between Japan and Vietnam) in order to realize a free and open Indo-Pacific," Suga said, according to the Japanese Foreign Ministry. The two countries will mark the 50th anniversary in 2023 of the establishment of their diplomatic relations.

The phone talks come as China steps up its claim over the Senkaku Islands, a group of East China Sea islets administered by Japan. Beijing has also militarized outposts in the South China Sea disputed by Vietnam and other neighbors.

The talks, requested by Vietnam, saw Suga and Chinh express possible cooperation in sectors such as infrastructure and energy, as well as in the environmental and digital fields.

The two leaders also reaffirmed working toward resolving North Korea's past abductions of Japanese nationals, among other issues concerning the country.