BRUSSELS—NATO’s new leader pledged continued support for beleaguered Ukraine in its fight against Russia’s invasion and vowed to put pressure on China to curtail its support of Moscow, whoever wins the U.S. presidential election.

The voice of support for Kyiv comes as it faces setbacks in both warfare and diplomacy, heading toward winter with much of the country’s energy infrastructure in ruins.

Mark Rutte , who took over from Jens Stoltenberg as secretary-general of the North Atlantic Treaty Organization on Tuesday, praised both U.S. presidential candidates for their commitment to defending Ukraine.

“I am absolutely convinced that on this issue, they both see what is necessary,” said Rutte, who in July ended 14 years as Dutch prime minister. Speaking at his first press conference in his new position, Rutte said defending Ukraine is one of his top priorities, alongside boosting NATO’s capabilities and “addressing the growing global challenges to Euro-Atlantic security.”

Rutte takes charge of the 32-country military alliance at a difficult moment for Ukraine, which faces increasing pressure from Russian advances in its east and signs of waning support from Western allies . Ukrainian President Volodymyr Zelensky last week visited Washington and New York to rally backing for his plans to make gains against Russia but was greeted with a lukewarm response .

Ukrainian forces in late summer invaded Russia, seizing portions of the Kursk region , but the brash move hasn’t shifted Kyiv’s fortunes as leaders had hoped. Russia, meanwhile, has battered Ukraine’s energy infrastructure, staged repeated attacks on civilian sites and made incremental battlefield advances at several points in eastern Ukraine.

“It’s not easy. It’s difficult,” Rutte said of Ukraine’s battlefield situation. He said Moscow is “making some limited gains,” though at the cost of an estimated 1,000 Russian troops killed or wounded daily.

“Supporting Ukraine is the right thing to do. And it is also an investment in our own security,” said Rutte. The Netherlands has been more directly affected than many NATO countries by Russia’s attacks on Ukraine, which began in 2014. That year, Malaysia Airlines Flight 17, which had departed Amsterdam for Kuala Lumpur, was shot down by forces aligned with Moscow, killing all 298 aboard.

“I know from personal experience, with the downing of flight MH17, how the conflict in Ukraine is not contained to the front lines,” Rutte said.

Rutte said that he was confident the Republican presidential candidate, former President Donald Trump , would stand by Ukraine despite Trump’s comments suggesting he wouldn’t. Rutte noted that he had worked with Trump for four years when they were both national leaders. He said that Trump had been correct to push other NATO members to boost military spending and stand up to China.

Rutte said Democratic candidate Kamala Harris had also voiced strong support for Ukraine.


China, Rutte said, “is an enabler of Russia’s brutal war of aggression.” He said that while still Dutch prime minister in March, he had visited China and told leaders there that their support for Moscow “has got to stop.” He said China is delivering microelectronics, raw materials and dual-use goods to Russia, circumventing international sanctions.

Rutte said NATO will increase cooperation with Indo-Pacific allies Japan, South Korea, Australia and New Zealand, including for the first time inviting their defense ministers to a regular gathering of NATO defense ministers later this month.

Write to Daniel Michaels at Dan.Michaels@wsj.com