SINGAPORE -- Authorities in Singapore on Friday stepped up their complaints over "intrusions" by Malaysian ships into what they called the city-state's territorial waters as tensions rose over port boundaries in one of Asia's busiest shipping lanes.

The Malaysian government later proposed seeking a resolution of the maritime boundary issue, but the Singapore side rejected a call for restraint.

Singapore Defense Minister Ng Eng Hen said in a Facebook post that Malaysian vessels' recent activities are "serious violations of Singapore's sovereignty."

"Our security forces have acted with restraint despite continuing intrusions and provocations," Ng added. "Singaporeans are peace-loving, but I strongly caution violators to leave Singapore territorial waters."

This followed a similar warning from Transport Minister Khaw Boon Wan on Thursday.

"Singapore will not hesitate to take firm action against intrusions and unauthorized activities in our waters," Khaw said in a statement. "My main message today is ... back off. Leave our waters while we pursue sit-down dialogues and try to resolve [the dispute]."

The flare-up began after Malaysia in October extended the boundary of a port in the southern city of Johor Bahru.

Malaysian media on Friday reported that Malaysian Foreign Minister Saifuddin Abdullah had proposed for both countries to stop sending vessels into the disputed area from midnight of Saturday. He said Malaysia had also proposed a meeting aimed at an amicable resolution of maritime boundary issues between the two countries, according to the report.

In response, Singapore's Foreign Ministry issued a statement disagreeing with the proposal to hold off on sending ships into the disputed area.

"Malaysian government vessels should forthwith cease these provocative violations of Singapore sovereignty and return to the status quo ante before 25 October 2018 [when Malaysia extended its port limits], without prejudice to our respective positions on maritime boundary claims in the area," the statement said. "Attempts to create facts on the ground add nothing to Malaysia's legal case and are unhelpful for an amicable resolution of our maritime boundary issues."

According to Khaw's statement, there were 14 "intrusions" into Singapore's territorial waters by Malaysian vessels from Nov. 24 to Dec. 5. Khaw described that action as Malaysia claiming territorial waters that belong to Singapore.

Malaysia's Prime Minister Mahathir Mohamad, center, foreground, meets Singapore's Prime Minister Lee Hsien Loong in November: The neighbors have traded accusations over control of waters between them.   © AP

Khaw also said Singapore's Prime Minister Lee Hsien Loong raised the issue during a summit with his Malaysian counterpart, Mahathir Mohamad, in mid-November, saying it could affect the bilateral relationship. "Unfortunately, further developments since then have made the situation more serious," Khaw said.

Singapore said the same day that it has decided to extend the boundary of its port in the western part of the city-state, putting it within its territorial waters.

It also released a video taken by the Defense Ministry that shows a white-hulled vessel, apparently belonging to a Malaysian agency, sailing in what it said were its waters.

Malaysian Transport Minister Anthony Loke earlier this week rebutted Singapore's claim. On Wednesday, Loke released a statement saying the new boundary for the Johor Bahru port "has not in any way encroached into any part of Singapore." He stated that Singapore has in recent years carried out extensive land reclamation in the area, but that Singapore's territorial waters remain unchanged.

Malaysia is also pushing to regain control of airspace in an area bordering Singapore that the latter has administered for decades.

"Malaysia and Singapore are like twins, except maybe the elder twin is a little bit bigger than the younger one," Mahathir posted on his Facebook page on Thursday. "As with most countries," he wrote, "there will be differences and there will be competition between us. However, it will help us grow even faster."

The two countries got along well during the previous government of Malaysian Prime Minister Najib Razak. But since Mahathir took over the reins in May there have been a number of irritants, including a dispute over water supplies from Malaysia and postponed plans for a high-speed railway between Singapore and Kuala Lumpur.

"There are no indications that there is domestic pressure from the people [in Malaysia] to the extent that the Mahathir government had to resort to foreign policy issues," said Norshahril Saat, a fellow at the ISEAS-Yusof Ishak Institute in Singapore. "It is rather too early to say how this incident will play out, but I am confident these issues will be resolved through diplomatic channels," Saat said.