A US ally spotted a small fleet of Chinese ships off the coast of highly contested islands on Monday, one of which contained a suspected autocannon.
Japan's coast guard reported the sighting near the Senkaku Islands - Japanese territories in the East China Sea. However, China is currently embroiled in a dispute over the islands as it seeks to extend its control over the region.
Four coast guard ships were seen in a strip of water 12-24 nautical miles from the shores of the islands, known as the contiguous zone.
Japanese authorities did not reveal which of the ships held the suspected autocannon - a fully automatic gun capable of rapid-firing large-caliber armour-piercing, explosive or incendiary shells.
On Friday, Japan also reported four Chinese ships in its waters off the coast of the islands. This came shortly after Joe Biden affirmed his commitment to defending the Senkakus from an attack.
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Japanese Prime Minister Fumio Kishida called China "an unprecedented and the greatest strategic challenge" to the world in a joint-session of Congress.
Following a meeting between the two leaders, a White House readout said: "Today, President Biden reaffirms the ironclad US alliance commitments to both Japan and the Philippines."
However, China still views the islands, which it calls the Daioyu Islands, as its own territory, and accused the US and Japan of spreading lies stating otherwise.
The Chinese Embassy in Tokyo said: "The Diaoyu Islands are China's territory. Whatever Japan and the United States say and whatever they do cannot alter these absolute facts.
"China's positions and propositions on the East China Sea and South China Sea issues have a full historical and legal basis."
China has tried to assert sovereignty over the Senkakus for years, maintaining a near-constant presence in the area. As of today, it has patrolled the islands for 116 days since December, according to data from Japan's coast guard.