Ishiba's first trip to Vietnam, and his subsequent visit on Tuesday to the Philippines, mark the latest high-level East Asian meetings amid escalating global uncertainty triggered by the threat of crippling US tariffs.

"The world economy is becoming more uncertain, and the impact on the Southeast Asian region is also becoming apparent," Ishiba told reporters on Monday after a meeting with Vietnam's Prime Minister Chinh.

In recent weeks, Vietnam has hosted China's President and top South Korean ministers, while Tokyo has held a trilateral meeting with China and South Korea.

Early in April, the White House slapped "bilateral tariffs" of 46 percent on Vietnam and 24 percent on Japan. Those duties were later paused until July as bilateral talks are underway, but a 10 percent levy applies on all imports into the United States, which is a major market for both countries. 

"We will cooperate to maintain a free and open international order based on the rule of law," Ishiba added in a joint press conference with Chinh where journalists' questions were not allowed.

Vietnam is a major assembling hub for large Japanese manufacturers, including Honda, Canon and Panasonic, with a total of $78 billion invested in the Southeast Asian country by Japanese firms, according to Vietnam's finance ministry. Japanese banks also hold strategic stakes in top Vietnamese lenders.

"The two countries agreed to uphold the global order of free trade based on international rules," Chinh said, after the two leaders signed four cooperation agreements, including on boosting trade in energy transition products and on research and development on semiconductors.

The content of the agreements was not disclosed and Reuters could not establish whether they entailed any binding or financial commitment.

Vietnam's top leader To Lam urged Japan to increase investment in infrastructure projects after he met Ishiba on Sunday, according to a report on Vietnam government's portal.

Japan has been involved in preliminary studies for the future construction of a high-speed railway connecting Hanoi to the southern business hub of Ho Chi Minh City, which with an estimated cost of $67 billion is Vietnam's most ambitious infrastructure project.

Leaders, however, did not explicitly mention railways among fields of cooperation. Vietnam has said it plans to finance the project largely on its own.