The Philippines has declared that war is not an option in resolving its dispute with China, which was found to have continued its military build-up in disputed South China Sea areas, declared by a UN arbitration court to belong to Manila.

President Rodrigo Duterte's office said on Monday that Filipino officials "know about the work" of China in seven disputed reefs, and the country relies "on the principle of good faith" that Beijing "will not reclaim new artificial islands".

A news report published on Monday said China "is almost finished" transforming the reefs into artificial island fortresses, complete with naval and air facilities, including runways and helipads.

The Philippine Daily Inquirer also published photographs showing radar domes, lighthouses, and hangars in the facilities spanning tens of thousands of square metres.

One of the areas, called the Mischief Reef, is within the Philippines' 370-km exclusive economic zone (EEZ), according to the Permanent Court of Arbitration in The Hague.

The report said the Philippines could lose 40 percent of its fishing grounds and 80 percent of its EEZ in the South China Sea, if it allows China's expansion to go unchallenged.

"Those islands were reclaimed even at the time of the former administration," Harry Roque, Duterte's spokesman, told reporters.

"I think whether or not we like it, they intended to use them as military bases," he said, adding there is nothing new in the news report.

"We cannot declare war. Not only is it illegal but also because it is impossible to declare war at this point."

Paolo Aquino, an opposition senator, raised concerns over the latest development and called on the Duterte administration to tell the public "what it gave up to China.

"While their warships are in our seas, we continue to give in to their whims and, all the while, we are kept in the dark as to our government's dealings with China," he said in a statement to Al Jazeera.

In July 2016, The Hague arbitration court ruled "there was no legal basis" for China to claim historic rights across large swaths of the South China Sea.

The case was filed by Duterte's predecessor, Benigno Aquino, after the Philippines found out that China had started stationing naval facilities in the resource-rich area.

'Don't trust a thief'

Duterte, who was eager to repair relations with Beijing, downplayed the ruling and said he "will not impose anything on China".

During his visit to China months after becoming president, Duterte also declared he has "realigned" himself with China's "ideological flow".

In exchange, China promised Duterte there will be no further expansion in other reefs, and committed to pouring billions of dollars in loans and investments into the country.

China claims the entire South China Sea, saying the Chinese people have been active there for more than 2,000 years.

Aside from the Philippines and China, neighbouring countries Malaysia, Brunei, Vietnam and Taiwan also have competing claims over parts of the South China Sea.

In November 2017, China and the Association of Southeast Asian Nations (ASEAN) announced they agreed to start talks on regional guidelines for the disputed territory, after 15 years of negotiations.

Roque said if there was "militarisation" in the South China Sea, it did not happen during the current administration.

"It's long been militarised," he said. "The question is what can we do? Right now, the posture of the president is to maintain close ties, so that they [China] won't have any reason to use those arms in those islands."

Florin Hilbay, the former government lawyer who argued the case at The Hague, denounced the Duterte administration's decision not to confront China on the issue of expansion and militarisation.

In a social media post on Monday, Hilbay wrote there are "offences to the nation that cannot be paid.

"The privileges of power are so fleeting compared with the enduring judgment of history and the moral indictment of fellow citizens," he said.

Antonio Carpio, a justice on the Philippine Supreme Court, also warned the Duterte administration of relying on China's promise not to reclaim other reefs.

"You don't rely on the good faith of the thief [who's trying to break] into your house," Carpio was quoted by the Philippine Daily Inquirer as saying.