Washington (CNN) The State Department has requested that US Marines be sent to Taiwan to help safeguard America's de facto embassy there, two US officials tell CNN, prompting China to urge the US to "exercise caution."

One US official said that while the request for a Marine security guard was received several weeks ago, it has not yet been formally approved and coordination about its deployment is ongoing between the State Department's Diplomatic Security Service and the Marines.

If the request is granted, it will be the first time in nearly 40 years that US Marines will be guarding a diplomatic post in Taiwan.

A spokesperson for the State Department would not say whether the request had been made, telling CNN, "We do not discuss specific security matters concerning the protection of our facility or personnel."

When asked about the potential deployment of Marines to Taiwan at a news conference Friday, Chinese Foreign Ministry spokesman Lu Kang said the US should exercise caution.

"That the US strictly abides by its 'one China' pledge and refrains from having any official exchanges or military contact with Taiwan are the political preconditions for China-US relations," Lu said. "The US is clear about the Chinese position and knows it should exercise caution on this issue to avoid affecting overall bilateral ties."

News of the official request for Marines comes just days after Secretary of Defense Jim Mattis wrapped up his first trip to Beijing , the first by an American defense secretary since 2014. Mattis met with Chinese military and civilian leaders, including President Xi Jinping.

As part of its "One China Policy," Washington does not formally recognize self-governing Taiwan as an independent country and therefore the US does not have an official embassy there.

However the US maintains a de facto embassy in the form of the American Institute in Taiwan which was established as part of the Taiwan Relations Act of 1979, which governs America's relationship with the island since the US switched its recognition to Beijing.

Washington's " One China Policy " differs from that advocated by Beijing, and the US takes no official position on the future status of Taiwan other than to oppose the unilateral changing of the status quo by either side.

Not 'even one inch'

"This is an important time in the history of China and the United States as we work our relationship forward," Mattis said while speaking alongside Xi prior to their meeting.

In their public remarks, neither Mattis nor Chinese officials made any direct mention of the issue of Taiwan, though Chinese state media reported that Xi did say that his country "cannot lose even one inch of the territory left behind by our ancestors," a statement that could be seen as a reference to Taiwan.

Two senior defense officials told CNN Wednesday that the Chinese raised the issue of Taiwan "multiple times" and "expressed their concerns" during their meetings with Mattis, citing recent US moves like the March passage of the Taiwan Travel Act, which encourages visits between officials of the United States and Taiwan at all levels.

The officials added that Mattis told his Chinese counterparts that he was not giving any "direction to military components to do anything differently" with regard to Taiwan.

"It wasn't an area that we wanted deep discussion on because we expect it to be an irritant," the officials said.

On Tuesday, Kang warned Taiwan against efforts aimed at separating itself from Beijing. The official said nothing, including the presence of foreign military forces, would stop Beijing from bringing the island under its rule.

"It won't work, even if they try to bank on foreign forces to build themselves up," he said.

China and Taiwan have been governed separately since the end of the Chinese Civil War in 1949.

Although both Beijing and Taipei view the island as part of "China" -- a term open to each side's interpretation -- neither government recognizes the legitimacy of the opposing side.

The Communist government in Beijing considers Taiwan a renegade province that must be reunited with the mainland, by force if necessary.

The Chinese military recently conducted a large scale military exercises that analysts saw as being aimed at taking the island by force.

Significant symbolism

The institute "undertakes a wide range of activities representing US interests, including commercial services, agricultural sales, consular services, and cultural exchanges," according to the US State Department.

On Wednesday the Institute announced that career American diplomat, William Brent Christensen, would take over as the director of the Institute's Taipei office in the summer of 2018, becoming the de facto US ambassador to Taiwan.

China lodged a protest with the US following the official opening of the Institute's new $255 million facility earlier this month, with China's Foreign Ministry slamming the US for allowing State Department representatives to attend the facility's opening ceremony.

One US military official said the request for US Marines was to help safeguard the new facility, which will house some 450 staff and sits in the outskirts of Taiwan's capital, Taipei.

The number of Marines posted to Taiwan is not expected to be large, likely less than 10. But the deployment carries significant symbolism as Marine guards have historically only been sent to countries with which the US has formal diplomatic relations.

US troops have not been permanently stationed in Taiwan since 1979, the year the US switched its formal recognition of China from Taipei to Beijing.

However, US Indo-Pacific Command, which oversees US military forces in the region, says that through the American Institute in Taiwan, the US maintains "a robust security cooperation program that includes arms sales, as well as maintenance, training, and exchanges."

Despite the lack of formal diplomatic ties, Taiwan remains an important American ally in the region, and under President Donald Trump, close, unofficial ties between the US and Taiwan have grown stronger.

Trump made headlines when he became the first American President-elect to accept a congratulatory phone call from Taiwanese President Tsai Ing-wen.

"As free and open democracies, we have an obligation to work with one another to defend our values and protect our joint interests," Tsai said at the ceremony marking the opening of the new American Institute.

"As long as we stand together, nothing can come between us," she added.

'Steadfastly committed'

The US also continues to sell Taiwan advanced weapons to aid in the island's defense as part of the requirements under the Taiwan Relations Act, actions that are met with condemnation by Beijing.

"The Department of Defense remains steadfastly committed to working with Taiwan to provide the defense articles and services necessary to maintain sufficient self defense consistent with our obligation set out in our Taiwan Relations Act," Mattis told an audience in Singapore earlier this month.

Tensions between the US and China have increased in recent weeks as the US has repeatedly slammed China for installing military facilities and equipment, including anti-ship and anti-aircraft missiles, on artificial islands in the South China Sea.

China claims sovereignty over the vast majority of the South China Sea, claims rejected by the US and the majority of countries in the region.