SEOUL - Around 20 South Korean protesters broke into the residential compound of the U.S. ambassador to South Korea Friday, prompting U.S. officials to call for tighter security measures around diplomatic missions here.

Video of the break-in posted online shows a group of young, chanting protesters using ladders to scale the stone wall surrounding Ambassador Harry Harris’ house, which is in a central area of Seoul.

After scaling the compound walls, the intruders attempted to forcibly enter the ambassador’s residence but were detained by Seoul police, according to a statement by the U.S. Embassy issued Saturday.

Some of the protesters carried signs calling for Harris to leave Korea and characterized U.S. troops as an occupying force.

Protesters shout slogans while holding signs to oppose planned joint military exercises between South Korea and the United States near the U.S. embassy in Seoul, South Korea, Aug. 5, 2019.

Pockets of anti-US sentiment

Although polls show South Koreans overwhelmingly support the alliance with Washington, pockets of anti-U.S. sentiment remain.

In 2015, a knife-wielding South Korean man with a history of militant Korean nationalism ambushed then-U.S. Ambassador Mark Lippert outside a building in downtown Seoul. Lippert sustained cuts to his arm and face.

More sporadic, minor disturbances have occurred since then.

“We note with strong concern that this is the second instance of illegal entry into the ambassador’s residential compound in 14 months,” a U.S. embassy official in Seoul said Saturday. “We urge the Republic of Korea to strengthen its efforts to protect all diplomatic missions to the ROK.”

The group that broke into the compound Friday says it is a coalition of progressive college students. Reuters reports the group recently held a forum to present “research findings” on the achievements of North Korean leader Kim Jong Un, and said it would welcome a visit by Kim to Seoul.

The students also attempted to break into the U.S. Embassy in Seoul last January before being stopped by police, Reuters reported.

Seoul’s foreign ministry said attacks on diplomatic facilities will not be tolerated, adding it will take “all appropriate measures” to prevent further incidents. Seoul police said they will increase security around the U.S. Embassy, according to the Yonhap news agency.

South Korean protesters hold banners during a rally as police officers stand guard near the Foreign Ministry in Seoul, South Korea, Feb. 10, 2019. South Korea and the United States are negotiating how much Seoul should pay for U.S. military presence.

Cost-sharing talks

The break-in comes at a particularly tense moment for U.S.-South Korea relations. The two countries next week will begin a second round of contentious negotiations over how to split the cost of the U.S. military presence in South Korea.

President Donald Trump has long complained that U.S. allies, and South Korea in particular, are not paying their “fair share” for the cost of U.S. troops.

Trump Says South Korea to Pay 'Substantially More' for US Troops Seoul hasn’t confirmed Trump’s announcement

In an apparent hardball negotiating tactic, Trump in August said South Korea agreed to pay “substantially more” for protection from North Korea. Seoul shot back, saying cost-sharing talks haven’t even begun.

South Korean reports say U.S. negotiators are demanding a fivefold increase in how much South Korea pays for U.S. troops. Harris appeared to indirectly confirm that figure in an interview last week.

He told the Dong-A Ilbo newspaper that from the U.S. perspective, South Korea could be seen as having funded only one-fifth of the total defense cost, and that as the world’s 12th-largest economy South Korea should take on a larger share.

South Korean officials have reportedly rejected the demand, saying they are prepared to engage in “reasonable” negotiations before the current cost-sharing agreement expires at the end of the year.

FILE - Protesters march after a rally to oppose a planned visit by U.S. President Donald Trump in Seoul, South Korea, June 29, 2019.

Anti-US displays rare

Over the past decade, overt displays of anti-U.S. sentiment have become less common in Seoul than in previous decades.

According to a 2018 Pew Research poll, 80% of South Koreans have a favorable view of the United States. That same poll, however, suggested just 44% of South Koreans have confidence in Trump.

Historically, conservatives have been the most reliably pro-U.S. contingent in South Korea. Recently, though, there has been a small backlash against Trump among conservatives, many of whom are already skeptical of Trump’s outreach to North Korea.

The situation has been exacerbated by Trump’s comments on cost-sharing negotiations. Trump reportedly recently used an Asian accent to mock South Korea’s president over the issue. Earlier this year, Trump said a certain country, widely seen as South Korea, was “rich as hell and probably doesn’t like us too much.”

The Pentagon says roughly 28,000 troops are in South Korea to help deter North Korean leader Kim Jong Un.

South Korea rejects Trump’s notion that it doesn’t contribute enough toward the cost of the U.S. troops, insisting it pays almost half of the total cost of $2 billion. That doesn’t include the expense of rent-free land for U.S. military bases, Seoul says.