Tokyo was preparing for the call on Tuesday evening local time, as diplomatic efforts for a peaceful resolution step up after an apparent opening from Russia, and with German Chancellor Olaf Scholz due in Moscow later in the day.

Kishida said Tuesday that Japan was "watching the situation with grave concern".

"We'll continue to monitor the situation with high vigilance, while coordinating closely with G7 countries to deal with any developments appropriately," he said at a meeting between government ministers and the ruling Liberal Democratic Party.

On Monday, a statement by finance ministers from the G7 group of most developed nations warned that Russia faced tough sanctions if it proceeded with military action against Ukraine.

The allies stood ready to "collectively impose economic and financial sanctions which will have massive and immediate consequences on the Russian economy", the statement said.

Japan's Foreign Minister Yoshimasa Hayashi on Tuesday stressed Tokyo's support for "the integrity of Ukraine's sovereignty and territory", but declined to give details on what form any sanctions might take.

"If a Russian invasion occurs, Japan will deal with it appropriately, including through imposing sanctions, in response to what has actually happened and in cooperation with the international community including the G7," he told reporters.

Japan's defence minister meanwhile warned of an increased Russian naval presence in the Sea of Japan and the southern part of the Sea of Okhotsk.

"It is thought that they intend to show off the capability to operate in the East and West, along with the Russian military's recent movement around Ukraine," Defence Minister Nobuo Kishi said in a tweet.

Japan's relations with Russia are complicated: the two countries never signed a peace treaty after World War II because of a lingering dispute over four islands claimed by Moscow in the closing days of the conflict.

The islands, off the northern coast of Japan's northernmost island of Hokkaido, are known as the southern Kurils in Russia and the Northern Territories in Japan.