Ukraine’s navy reported destroying Russia’s large landing ship, Orsk, near the port city of Berdyansk.
The US formally declared that Russia has committed war crimes in Ukraine.
Nato officials say at least 7000 Russian soldiers have been killed in the past month, and it could be up to 15,000.
Ukrainian President Volodymyr Zelenskyy called for people around the world to show support for Ukraine on Thursday, which will mark the start of the war’s second month.
US President Joe Biden is in Europe for emergency Nato and EU talks in Brussels. He will visit Poland afterwards.
Ukraine’s navy on Thursday reported destroying Russia’s large landing ship, Orsk, near the port city of Berdyansk.
A short Facebook statement about the ship was accompanied with photos and videos of fire and thick plumes of smoke in the port.
The Russian military has not commented on what happened to the ship.
Berdyansk has been under Russian control since February 27.
Asked whether a chemical weapons attack is a red line for Nato, Stoltenberg said, “I will not speculate beyond the fact that Nato is always ready to defend, to protect and to react to any type of attack on a Nato-allied country.”
Stoltenberg says “any use of chemical weapons would fundamentally change the nature of the conflict. It would be a blatant violation of international law, and it will have widespread and severe consequences.”
His remarks Thursday came as he arrived at Nato headquarters in Brussels to chair a summit of the military organization’s 30 national leaders, including US President Joe Biden.
Nato allies are worried about Russian rhetoric and fears that Moscow might want to create a pretext to use chemical weapons in Ukraine. The leaders are likely to agree to send equipment to help Ukraine protect against chemical, biological, radiological and nuclear threats.