Reuters/New Delhi

India summoned its ambassador to Nepal home for urgent talks, officials said on Tuesday, following a sudden downturn in ties after its neighbour adopted a new constitution that sparked weeks of border violence that killed more than 40 people.

The new strain in relations with Nepal, a Himalayan nation that acts as a buffer state with China, threatens to undermine Prime Minister Narendra Modi's regional diplomacy push aimed at countering Beijing's expansion of influence in South Asia.

Nepal's new charter took effect on Sunday, despite fierce protests from minority groups in the southern plains whose homeland provinces are to be split up.

India's ambassador to Nepal, Ranjit Rae, arrived in New Delhi on Monday for day-long consultations, Indian foreign ministry officials in New Delhi and Kathmandu said.

In a statement, India's foreign ministry criticised Nepal over the unabated violence in its southern region, where police shot at least three protesters on Monday.

"We had repeatedly cautioned the political leadership of Nepal to take urgent steps to defuse the tension in these regions," the foreign ministry said in the statement late on Monday. "This, if done in a timely manner, could have avoided these serious developments."

The comments highlight a deterioration in ties after India's swift response in aid efforts for Nepal this spring, following two devastating earthquakes that killed more than 9,000 people.

Indian freight companies and transporters moving goods through Nepal's southern plains have also complained about  security in the area, the statement added.

India and landlocked Nepal share an open border and are bound by long-standing economic, cultural and ethnic ties. But Nepal has frequently railed against what it calls meddling in its affairs by its large southern neighbour.

In an editorial, Nepal's leading English language daily, the Kathmandu Post, acknowledged the failure of efforts to reconcile opposition groups to the constitution, but warned India not to overstep the line of diplomacy.

"India could box itself in a difficult position and lose its diplomatic leverage against certain parties and sections of the polarized society," the paper said.

By Tuesday morning, the Twitter hashtag #BackOffIndia was trending in Nepal.

Nepal's new constitution creates seven states in a federal system, but is opposed by groups who want to re-establish Nepal as a Hindu nation, and others who feel it is unfavourable to people in the plains.

China has welcomed the new charter.