China says India has been building up troops along its side of the border and demands an "immediate withdrawal" amid an increasingly tense standoff in a remote frontier region high in the Himalayas.

The standoff, on a plateau that lies at the junction between China, India, and Bhutan, is one of the worst border disputes between the nuclear-armed rivals in more than 30 years.

On Thursday, China's foreign ministry demanded India must "immediately withdraw troops" from the region, known as Doklam in Bhutan and India, and Donglang in Beijing, if it cherishes peace.

The plateau is currently disputed between China and Bhutan with India supporting Bhutan's claim over it.

"It has already been more than a month since the incident, and India is still not only illegally remaining on Chinese territory, it is also repairing roads in the rear, stocking up supplies, massing a large number of armed personnel," China's foreign ministry said in a statement.

"This is certainly not for peace."

READ MORE: China demands India pulls back troops in border dispute

India, meanwhile, denied any such military buildup.

In a statement to parliament on Thursday, Foreign Minister Sushma Swaraj urged dialogue based on a written common understanding regarding the border intersection reached in 2012.

"India always believes that peace and tranquility in the India-China border is an important pre-requisite for smooth development of our bilateral relations," Swaraj said, according to a transcript of her remarks released by her office.

"We will continue to engage with the Chinese side through diplomatic channels to find a mutually acceptable solution."

'Bottom line'

The standoff began in June when Chinese troops began building a road through the plateau.

India sent troops at Thimpu's request and warned China that construction of a road near their common border would have serious security implications.

The road gives China access to the so-called Chicken's Neck, a thin strip of land connecting India's central mass to its remote northeastern regions.

Indian officials say about 300 soldiers from either side are now facing each other about 150 metres apart.

In a separate statement, China's defence ministry said the country had shown goodwill and that its forces had exercised utmost restraint, but warned, "restraint has a bottom line".

"No country should underestimate the Chinese military's confidence in and ability to fulfil its mission of safeguarding peace, and should not underestimate the Chinese military's determination and will to defend the country's sovereignty, security and development interests," it said.

China's military has held live fire drills close to the disputed area, and state television on Friday said more exercises had been conducted recently, though did not give an exact location.

In an editorial, the official China Daily said China was not in the mood for a fight, noting how the standoff has been "unusually restrained".

"However, if good manners do not work, in the end, it may be necessary to rethink our approach. Sometimes a head-on blow may work better than a thousand pleas in waking up a dreamer," the English-language paper said.

Narendra Modi, Indian prime minister, is to visit China early in September for a summit of BRICS leaders.