What's behind the stand-off?

Indian military officials told regional analyst Subir Bhaumik that they protested and stopped the road-building group, which led Chinese troops to rush Indian positions and smash two bunkers at the nearby Lalten outpost.

"We did not open fire, our boys just created a human wall and stopped the Chinese from any further incursion," a brigadier said on condition of anonymity because he was not authorised to speak to the press.

Chinese officials say that in opposing the road construction, Indian border guards obstructed "normal activities" on the Chinese side, and called on India to immediately withdraw.

What is the situation now?

Both India and China have rushed more troops to the border region, and media reports say the two sides are in an "eyeball to eyeball" stand- off.

The Chinese ambassador to India Luo Zhaohui told Press Trust of India news agency on Tuesday that India had to "unconditionally pull back troops" for peace to prevail. The statement is being seen as a diplomatic escalation by China.

Bhutan, meanwhile, has asked China to stop building the road, saying it is in violation of an agreement between the two countries.

What does India say?

Indian military experts say Sikkim is the only area through which India could make an offensive response to a Chinese incursion, and the only stretch of the Himalayan frontier where Indian troops have a terrain and tactical advantage.

They have higher ground, and the Chinese positions there are squeezed between India and Bhutan.

"The Chinese know this and so they are always trying to undo our advantage there," retired Maj-Gen Gaganjit Singh, who commanded troops on the border, told the BBC. Indian Defence and Finance Minister Arun Jaitley also warned that the India of 2017 was not the India of 1962, and the country was well within its rights to defend its territorial integrity.

What does China say?

China has reiterated its sovereignty over the area, saying that the road is in its territory and accusing Indian troops of "trespassing".

It said India would do well to remember its defeat in the 1962 war, warning Delhi that China was also more powerful than it was then.