NEW DELHI (REUTERS, AFP) - Diplomatic efforts to tackle the crisis caused by North Korea's nuclear and missile buildup are continuing, US Defence Secretary James Mattis said on Tuesday (Sept 26).

Mattis, who is on a two-day official visit to the Indian capital to strengthen military ties, said pressure on North Korea had increased following a United Nations resolution. "We continue to maintain the diplomatically led efforts in the United Nations," he said.

"You have seen unanimous UN security council resolutions passed that have increased the pressure... on the North and at the same time we maintain the capability to deter North Korea's most dangerous threats," he added.

Speaking to reporters after a bilateral meeting with his Indian counterpart Nirmala Sitharaman, Mattis also said that the US appreciates Indian and international efforts to step up pressure on North Korea over its "dangerous and destabilising behaviour".

He added there can be no tolerance for terrorist safe havens and pledged to work together with India to end the scourge. India has long accused Pakistan of harbouring militants on its soil.

"There can be no tolerance of terrorist safe havens. As global leaders, India and the United States resolve to work together to eradicate this scourge," he said in a statement. He also said Washington welcomed Indian efforts to help stabilise the situation in Afghanistan.

The Defence Secretary also spoke to Indian leaders about the potential sales of US jet fighters and surveillance drones that experts say are aimed at helping it rein in China's influence in the region.

Mattis is the first Cabinet rank official to visit India under the Trump administration and was due to meet Prime Minister Narendra Modi.

Ties between India and the US have rapidly expanded with New Delhi buying weapons worth US$15 billion (S$20.3 billion) over the last decade from the US, moving away from traditional supplier Russia.

At the top of the agenda is to move forward with a deal to supply 22 Sea Guardian drone aircraft to the Indian navy that the US government approved in June, the first such clearance to a non-NATO ally.

The Indian navy has sought the unarmed drones to help it mount longer duration surveillance of the Indian Ocean where Chinese naval ships and submarines are making regular forays.

The US has been critical of China's build-up of military facilities in the South China Sea and had suggested joint patrols with the Indian navy across the region. New Delhi has turned it down, fearing a Chinese backlash.

"China looms very large for both countries," said Dhruva Jaishankar, a specialist on India-US relations at Brookings India. "The strategic underpinning of India-US defence ties is in the common concerns they have over China, over its revisionism."

The Indian air force has also asked for 90 armed Avenger Predator drones that experts say it could deploy to conduct cross-border strikes, for instance on camps of militants that it says exist on the Pakistan side of disputed Kashmir.

Such a sale would need White House and Congress approval, sources said. Pakistan and possibly China are likely to see such a sale as de-stabilising.

Vivek Lall, chief executive, US and International Strategic Development at General Atomics, the company that makes the drones, said it was pleased the Indian government had won approval for the surveillance version of the drone.

"Not only will this platform enhance India's capabilities in the areas of maritime domain awareness and security - but the inter-operability with one of the US' most important strategic partners will contribute to security across the region," he said.

The two sides are also expected to discuss Lockheed Martin's offer to build F-16 fighter planes in India as part of Modi's drive to build a domestic military industrial base.

Sweden's Saab is the other contender for the deal to supply at least 100 single engine combat planes to the Indian air force that the Modi administration wants to be built locally.

Ahead of Mattis's trip, the Pentagon had stressed India's broader role in the region. "The secretary will emphasize that the United States views India as a valued and influential partner, with broad mutual interests extending well beyond South Asia," it said.