• The Philippines said it spotted a Russian submarine in the waters of the South China Sea last week.
• President Ferdinand Marcos Jr. said the sub's presence in Philippine waters was "very concerning."
• One analyst said it could be Russia signaling to the US that it will aid China amid regional tensions.

The Philippines said it spotted a Russian submarine in the South China Sea last week, in the latest sign of tensions in the contested waters.

In a Facebook post on Tuesday, the Armed Forces of the Philippines said a Russian UFA 490 submarine was seen 80 nautical miles from its coast last Thursday, prompting the Philippine Navy to deploy a plane and a warship to monitor the vessel and establish radio contact with it.

The Russian vessel responded, saying it was waiting for better weather conditions before heading to the port of Vladivostok, in Russia's Far East, per the post.

The Inquirer first reported the news, citing unnamed security sources, including one who said it was a Kilo II-class diesel-electric submarine.

President Ferdinand Marcos Jr. said on Tuesday the reported presence of the Russian vessel in the Philippines' Exclusive Economic Zone was "concerning."

He added that any "intrusions" into the West Philippine Sea were "very worrisome."

Last month, Russian state news agency TASS reported that a submarine and a rescue tug had made a port call in Malaysia before Russian and Malaysian navies were due to conduct joint drills in the South China Sea.

Rear Adm. Roy Vincent Trinidad, a Philippine Navy spokesperson, told reporters on Tuesday that a rescue tug and a support vessel were spotted near the sub.

He told Agence France-Presse that the incident was not "alarming" but that they were "surprised" because "this is a very unique submarine."

This latest incident comes as tensions in the South China Sea have escalated this year, especially between China and the Philippines, an ally the US is treaty-bound to defend.

This has included clashes between Chinese and Filipino coast guard vessels, including one where swords and knives were brandished.

Hunter Marston, an Asia-Pacific researcher at the Australian National University, said the submarine may have sailed through Philippine waters because it was the fastest route between Malaysia and Russia.

"Alternate routes involve circumnavigating the Philippine and Indonesian archipelagos or cutting directly through the Taiwan Strait," he told BI, which add their own geopolitical risks.

But other South China Sea observers said the vessel's presence in the waters could be a message.

Eduardo Araral, an associate professor at the National University of Singapore, told The Strait Times that the presence of a Russian submarine in the region could be meant as a signal to the US.

Araral said that it could be the Russians telling the Americans: "If you threaten us, we can threaten you back."

He added that news that Russian submarines were operating in the South China Sea was not surprising, given that Russia has been stepping up its participation in war drills with its Pacific allies.

Sari Arho Havrén, an associate fellow at the Royal United Services Institute specializing in China's foreign relations, went further.

She told BI that increased China-Russia cooperation is meant to make US regional allies and partners "increasingly anxious."

These actions also "demonstrate how China would not necessarily be alone should any crisis escalate," she added.

Russia conducted joint military drills with China in the South China Sea in July and September.

The Philippines' Armed Forces and the Philippine Navy didn't respond to requests for comment.