One observer noted that President Donald Trump's preoccupation with Russia could have led to the postponement.

External Affairs Minister Sushma Swaraj and Defence Minister Nirmala Sitharaman were to travel to Washington to take part in the meeting with US Secretary of State Michael R Pompeo and Secretary of Defence James Mattis on July 6.

This new dialogue format was agreed upon between the two sides during the visit of Prime Minister Narendra Modi to Washington in June 2017. It was seen as a vehicle to elevate the strategic relationship between the two countries with special focus on on bolstering strategic, security, and defence cooperation.

Safe to say that US-India relations have taken their "biggest tumble" in quite some time, tweeted Michael Kugelman from the Wilson Center.

"In the Trump era, no US relationship is foolproof," he said. "This is really big. 2+2 could have been used to reset a relationship experiencing growing tensions on the econ side," he said in another tweet.

A State Department spokesperson told PTI that External Affairs Minister Swaraj and her American counterpart Pompeo during a phone call on Wednesday agreed to reschedule the 2+2 dialogue as soon as possible at a mutually convenient time and location.

Pompeo spoke over phone with Swaraj on June 27 to discuss further strengthening US-India cooperation. Pompeo conveyed to Swaraj "his regret at the postponement" of the 2+2 dialogue previously scheduled to take place on July 6, the spokesperson said.   

"The Secretary and Minister Swaraj agreed the 2+2 dialogue would be rescheduled as soon as possible at a mutually convenient time and location," the State Department spokesperson said, hours after Ministry of External Affairs spokesperson Raveesh Kumar tweeted that during the call Pompeo expressed his "regret and deep disappointment" at the US having to postpone the 2+2 Dialogue for "unavoidable reasons."

Without explaining the "unavoidable reasons", the State Department Spokesperson asserted that the US-India relationship was "a major priority" for the Trump administration, and it looks forward to continuing to strengthen the partnership.

The postponement of the 2+2 talks is "unfortunate and more than a little embarrassing for the United States," said Joshua T White, a former Obama administration official.

"But frankly I'm more worried that the growing cascade of disputes surrounding trade and investment will slowly sap momentum from this very important relationship," White, currently a fellow at the Edwin O Reischauer Center for East Asia Studies at The Johns Hopkins University School of Advanced International Studies in Washington, told PTI.

After June last year, the two countries have tried to schedule the dialogue many times with several dates having been considered.

Earlier this year also, the '2+2 dialogue' had been postponed due to uncertainty over the confirmation of Pompeo as President Donald Trump's new Secretary of State. Pompeo was confirmed as Secretary of State in April.

The State Department spokesperson noted that "India's central role in US national security is enshrined in the President's National Security Strategy, which noted that "We welcome India's emergence as a leading global power and stronger strategic and defense partner."

The spokesperson noted that US Ambassador to the UN Nikki Haley is in New Delhi right now for a visit focused on advancing the US-India relationship.

The postponement of the 2+2 dialogue comes in the backdrop of India's plan to purchase the S-400 missile defence system from Russia, which was expected to be among the topics up for discussion.

The US passed the Countering America's Adversaries Through Sanctions Act (CAATSA) against Russia in August 2017 for reportedly influencing and manipulating the 2016 presidential election process. India wants its defence deals with Russia outside the purview of the CAATSA.

It also came a day after the Trump administration told India and other countries to cut oil imports from Iran to "zero" by November 4 or face sanctions, making it clear that there would be no waivers to anyone.

Bharat Gopalaswamy from the Atlantic Council said it was "not so pleasing" to see the 2+2 being called off.

"I believe the Trump administration is distracted with a summit with Russian President Vladimir Putin," he said.

"Like the summit with North Korea, they believe they might be able to advance their relations with Russia, whatever that means in tangible terms. The US relations with Russia have deteriorated significantly over the last 18 months and I believe the Trump might be able to turn around a corner with one of US's significant adversaries," he said.

"So as far as I can understand, this is a very distracted administration but Trump has always had a fascination for Russia, Mr. Putin and the US being able to strike some sort of a deal with Russia. And that's where he has set his focus right now," Gopalaswamy told PTI in response to a question.

"I also do not think buying arms would be a concern for Trump administration. If that were the reason, why would Trump be focused on a summit with Russia," he added.