The United States warned Thursday of a looming "humanitarian crisis" in quake-hit Nepal which is facing crippling shortages of vital supplies as protesters block a key checkpoint bordering India.

Scores of trucks have been stranded at a crossing in the southern town of Birgunj for nearly six weeks, cutting off supplies and forcing fuel rationing across the landlocked Himalayan nation.

In a crackdown that dashed hopes of a compromise between the government and demonstrators, Nepali security forces on Monday broke up the blockade for a few hours, sparking violent protests and leading to the death of an Indian man in police firing.

In a statement, the US embassy in Kathmandu said it was "deeply concerned by the increasingly volatile situation along the Nepal-India border, resulting in critical shortages of fuel, medicine, and foodstuffs".

"Lives are at stake, and we are concerned that a humanitarian crisis may result."

The blockade that began on September 24 has forced international aid organisations to halt relief operations for tens of thousands of quake victims just weeks before winter.

The protesters, from Nepal's Madhesi ethnic minority, want lawmakers to change the country's new constitution which they say will leave them politically marginalised and under-represented in parliament.

In the embassy statement, the US urged all parties to return to the negotiating table, joining Nepal's regional ally India in calling for talks to resolve the crisis.

The constitution, the first drawn up by elected representatives, was meant to cement peace and bolster Nepal's transformation to a democratic republic after decades of political instability and a 10-year Maoist insurgency.

But it has resulted in deadly violence with more than 40 people killed in clashes between police and protesters. The constitution was introduced in September after April's massive earthquake pushed warring political parties to reach agreement.