WASHINGTON/HONG KONG (REUTERS) - The United States is "deeply disappointed" by El Salvador's decision to break diplomatic ties with Taiwan in favour of China and is reviewing its relationship with San Salvador as a result, a spokesman for the US State Department said on Tuesday (Aug 21).

The comments came as signs indicate the US is worried tiny El Salvador has something more to offer mainland China than diplomatic recognition: a port that could be used for military purposes.

The Central American country officially cut ties with Taipei and established a formal alliance with Beijing earlier on Tuesday, leaving Taiwan with just 17 diplomatic allies.

El Salvador was the fifth country in the past two years to switch allegiances from Taipei to Beijing and said attracting investment and developing the economy were key goals behind the decision.

"Although we recognise the sovereign right of every country to determine its diplomatic relations, we are deeply disappointed by this decision," said a spokesman for the US State Department, who did not want to be identified by name.

"We are reviewing our relationship with El Salvador following this decision."

Washington sees the latest switch as not just a case of piling more pressure on self-ruled Taiwan - which the US is obliged to help maintain its defence capabilities - but also a move to shore up mainland China's security and strategic planning in the region, South China Morning Post reported on Wednesday.

US relations with El Salvador have also been strained over President Donald Trump's threats to cut aid from countries that "do nothing" to stop MS-13 gang members from crossing illegally into the United States.

Ms Jean Manes, the US ambassador to El Salvador, tweeted on Tuesday that the US was concerned about the Central American nation's decision to break ties with Taiwan.

"Without a doubt, this will impact our relationship with the government. We continue supporting the Salvadoran people," she said.

Last month, Ms Manes warned about China's intentions to turn the La Union commercial port in El Salvador's east into a "military base", according to US-based MintPress News.

"It is a strategic matter, and we all need to keep our eyes open to what is happening," she said.

El Salvador has an area half the size of Taiwan and is the smallest country in Central America. Its economy relies on exports of coffee, sugar, textiles and clothing, and the assembly of intermediate goods, and a third of its population of 6.5 million lives below the poverty line, SCMP said.

The shipping port of La Union has remained largely deserted since it was completed in 2008 because its lack of maritime traffic has made it difficult to find investors.

On Tuesday, Taiwan's Foreign Minister Joseph Wu said Taipei had rejected El Salvador's request for harbour development funding, without naming La Union, over concerns that the site was unsustainable.

But last month, El Salvador Economy Minister Luz Estrella Rodriguez said Beijing was interested in reviving the port, in addition to investing in other areas of the country, reported SCMP.

"It is perfectly natural that China would be interested in having a port in El Salvador - it's located at the centre of Central America, acting as a transport and trade hub linking North and South America," SCMP quoted Mr Xu Shicheng, a research fellow with the Institute of Latin American Studies at the Chinese Academy of Social Sciences, as saying.

"The US likes to say that it would be of military use just because it has a huge military presence in the region, but in fact, China has no military presence in the region," Mr Xu said.

China opened its first overseas military base in Djibouti last year, and is possibly developing one in Gwadar Port in Pakistan, but observers said it was unlikely it would be planning to build a military port in El Salvador.

Dr Collin Koh, a naval affairs expert at the S. Rajaratnam School of International Studies in Singapore, said it would be far-fetched to say Beijing saw ports as "potential bases", but it was clear that it was seeking to acquire or help develop overseas ports in strategic locations that could serve its long-term interests.

"To build a base in this region may run the risk of provoking a stronger US response, even as we can still entertain the idea of the occasional access to port facilities by the People's Liberation Army Navy," Dr Koh was quoted by SCMP as saying.

Last Africa ally

Taiwan vowed on Tuesday to fight China's "increasingly out of control" behaviour after El Salvador switched allegiance but President Tsai Ing-wen, who has vowed not to bow to Chinese pressure, came under opposition criticism on Wednesday amid calls for a more friendlier policy towards Beijing.

Ahead of next month's summit between China and African leaders in Beijing, China has been upping the pressure on Taiwan's last remaining ally on the continent, eSwatini, formerly known as Swaziland, to come over to China's side, diplomatic sources say.

Speaking to reporters in Beijing about the summit, Chinese Assistant Foreign Minister Chen Xiaodong that eSwatini did not have relations with China "for reasons that everyone knows".

"We look forward to and hope that all African nations, with none left behind, can take part in positive China-Africa cooperation, and become a member of the largest family get together," Mr Chen said. "I believe that this is not just the pursuit of China, it is also a widespread shared expectation of African nations. I believe that this target can in the not too distant future be realised," he added, without elaborating.

Taiwan's biggest opposition group, the China-friendly Kuomintang (KMT), urged for a "better alternative" following what it described as "enormous mistakes" in Ms Tsai's cross-strait policy.

"The ruling party has the responsibility to think of a better cross-strait policy... The relations with China have turned stagnant and were frozen," KMT's spokesman Hung Mong-kai told Reuters.

Mr Lin Yu-fang, a KMT lawmaker who leads the diplomacy and defence committee in the Taiwan parliament, urged Ms Tsai to recognise the "one China" principle, an agreement reached between Beijing and then-ruling KMT in 1992, under which both agreed there is only one China, with each having their own interpretation of what that means.

"She should bravely tell her supporters they need a friendlier policy to China, this way there would be more support for her," Mr Lin said.

China's hostility to Taiwan has grown since Ms Tsai's election, as Beijing fears she wishes to push for the island's formal independence, a red line for China. She says she wants to maintain the status quo but will defend Taiwan's democracy.

State-run Chinese newspaper the Global Times said in a Wednesday editorial that China did not have to "pay a fortune to steal Taiwan's 'allies'".

"Many of the island's 'allies' have a larger trade volume with China than with Taiwan. Their establishment of diplomatic ties with the Chinese mainland is an irresistible trend. It is only a matter of time before Taiwan has zero 'allies.'"