US Secretary of State John Kerry Sunday said the U.S. relationship with Pakistan "could not be more important" as Islamabad struggles with economic, security and regional stability issues.

Kerry spoke Sunday before he held talks with Prime Minister Nawaz Sharif, who is on an official visit to the United States.

The meeting between the Prime Minister and Secretary of state lasted for an intense three hours during which regional and international situation and issues pertaining to bilateral relations were discoursed at length, the sources said. Other matters of mutual interest including regional stability and situation in Afghanistan were also discussed during the dinner meeting.

From the US side, senior Administration officials including Defence Secretary Chuck Hagel, National Security Advisor Susan Rice, Director CIA John Brennan and SRAP James Dobbins assisted the Secretary of State.

At the outset of the dinner meeting before the media, Secretary Kerry warmly welcomed the Prime Minister saying Mr Nawaz Sharif is visiting Washington for the first time since his last trip in 1999.

John Kerry underscored Pakistan's vital significance to regional stability as the country addresses the challenges of economy and insurgency simultaneously.

He said Nawaz Sharif will have a series of high-level meetings‚ including discussions with President Barack Obama and Vice President Joseph Biden on Wednesday.

As a gesture of goodwill towards Pakistan, U.S. officials say the Obama administration is posed to release more than $1.6 billion in military and economic aid to help bolster ties with Islamabad that have deteriorated over deadly American airstrikes and the 2011 raid that killed Osama bin Laden in Pakistan.

"We have a lot to talk about, and the relationship with Pakistan could not be more important," Kerry said as his meeting with Sharif began.

"On its own, Pakistan is a democracy that is working hard to gets its economy moving and deal with insurgency, and also important to the regional stability" Kerry said.

Secretary Kerry expressed the confidence that the visit would provide the necessary impetus to further deepen and expand the US- Pakistan relations. Pakistan was an important country in the region and the US would be working with the democratic government of Prime Minister Muhammad Nawaz Sharif in all areas of bilateral relations under the revived mechanism of the Strategic Dialogue, the Secretary stressed. He further appreciated the bold decisions taken by Prime Minister Sharif for reviving the economy and overcoming the energy shortages.

Prime Minister Muhammad Nawaz Sharif noted that he looked forward to meeting with President Obama at the White House on 23 October. This would be the first summit level interaction between the two countries after the historic democratic transition in Pakistan. The Prime Minister reiterated his determination to revive the economy and improve the law and order situation in Pakistan. Outlining his vision of promoting peace and stability in the region, the Prime Minister emphasized the need to work together in meeting the common challenges of extremism and terrorism.

Meanwhile the US State Department has asked Congress to resume more than $300 million in blocked security assistance to Pakistan, officials said Sunday amid an upswing in relations.

"This is part of a long process of restarting security assistance cooperation after implementation was slowed during the bilateral challenges of 2011 and 2012," deputy State Department spokeswoman Marie Harf said.

US security assistance was interrupted during that period, although $857 million in civilian assistance continued to flow, Harf said.

She said US security assistance would build the capabilities of Pakistan s security forces, "which is critical to countering violence in the western border regions."

"And US civilian assistance to Pakistan has delivered real results on the issues most important to Prime Minister Sharif and all Pakistanis: energy, education, and economic growth," she said.

The Prime Minister was accompanied by Advisor to the Prime Minister on National Security and Foreign Affairs Mr. Sartaj Aziz, Finance Minister Mr. Ishaq Dar, Special Assistant to the Prime Minister on Foreign Affairs Syed Tariq Fatemi and Foreign Secretary Mr. Jalil Abbas Jilani.

Earlier, upon his arrival at the Andrews Air Force Base in Washington for a three day official visit, the Prime Minister was accorded warm welcome by the senior officials of the White House and the Department of State. A smart contingent of the US armed services also presented guard of honour to the Prime Minister.