The ministry said in a statement on Sunday that it had conveyed its concerns to the embassy without clarifying whether it had received a response.
The roughly 15-metre (49.21 ft) banner, in the colours of the Russian flag and written in Russian, was hung on the embassy’s outer wall in central Seoul ahead of the fourth anniversary of Russia’s invasion of Ukraine on Tuesday.
The banner remained in place on Monday.
In its statement, the ministry reiterated South Korea’s position that Russia’s invasion of Ukraine is illegal.
The ministry also said that military cooperation between Russia and North Korea should stop, describing it as a grave threat to South Korea’s security and a violation of the UN Charter and UN Security Council resolutions.
Earlier this month, Russian Ambassador to South Korea Georgy Zinoviev praised what he described as North Korean troops’ role in fighting in Russia’s Kursk region, according to media reports.
Under a mutual defence pact with Russia in 2024, North Korea sent some 14,000 soldiers to fight alongside Russian troops against Ukraine, where more than 6,000 of them were killed, according to South Korean, Ukrainian and Western sources.
The Russian embassy in Seoul could not immediately be reached for comment by phone. An automated voice message stated the embassy was closed due to a public holiday on Monday.