Japan has agreed to provide $4.5 billion in aid for Ukraine this year, part of a 10-year security deal signed by the two countries on Thursday, President Volodymyr Zelensky of Ukraine said.

The agreement significantly increases Japan’s support for Ukraine at a critical moment in its war with Russia and underscores Japan’s efforts to strengthen its security and diplomatic ties with Europe after the full-scale conflict began in 2022.

“For Japan, this type of agreement and this level of support is a breakthrough,” Mr. Zelensky said on social media. “We see this and thank Japan for its unwavering solidarity.”

Mr. Zelensky said Prime Minister Fumio Kishida of Japan had signed the pact as the leaders gathered for the Group of 7 summit discussed other plans to support Ukraine.

Ukraine’s deal with Japan included commitments on defense support, humanitarian aid and technical cooperation, Mr. Zelensky said.

Mr. Kishida said in a statement earlier this week that Japan would “do its utmost to ensure” that peace was restored in Ukraine.

Ukraine has signed several similar agreements with wealthy nations during the war.

Last December, Japan outlined a $1 billion assistance commitment for Ukraine, and signaled a willingness to later increase the support to $4.5 billion.

“The scale of the involvement is an escalation,” Daniel Sneider, a lecturer in East Asian studies at Stanford University, said of Japan. “They want to march in lock step with NATO. That’s a pretty big deal in Japan.”

Separately, President Biden was expected to sign a 10-year U.S. security agreement with Ukraine on Thursday, the first day of a three-day G7 summit in Italy.

The leaders of the G7 nations — Britain, Canada, France, Germany, Italy, Japan and the United States — also agreed on a plan to provide Ukraine with a $50 billion loan to help it buy weapons and restore infrastructure.