Washingtonn (CNN) A group of 19 humanitarian and human rights nongovernmental organizations are calling on US Secretary of State Rex Tillerson to use all diplomatic means at his disposal to end the blockade of aid to Yemen, warning that the war-torn nation faces a rapidly deteriorating situation that "puts millions of civilian lives at risk."

In a letter sent Monday to Tillerson, US Ambassador to the United Nations Nikki Haley, National Security Adviser H.R. McMaster and Defense Secretary James Mattis, the coalition of NGOs highlighted the increasingly dire humanitarian crisis in Yemen stemming from "almost 1,000 days of war."

It calls on the US and the international community to work toward achieving several specific objectives aimed at addressing the situation in Yemen:

Condemn all attacks on civilians by all parties, demand an urgent ceasefire by all parties to the conflict, and generate genuine commitments by Saudi Arabia and the United Arab Emirates to respect international humanitarian law and end deliberate or indiscriminate attacks on civilians;

Set forth a plan to reinvigorate peace talks and end unlawful attacks on civilians, through a realistic and concrete roadmap to peace.

"The United States bears a special responsibility to use all diplomatic means to end ongoing violations against civilians in Yemen and the recent blockade of the country, to obtain concrete commitments by all sides for unfettered humanitarian access, and to secure a ceasefire and resumption of the peace process," the letter said.

"The breakdown in Yemen of respect for international law and the rapidly worsening humanitarian situation puts millions of civilian lives at risk, and threatens not only regional stability but also international peace and security more broadly," it added.

Saudi Arabia tightened its blockade on Yemen after Houthi rebels, who have taken over the national government and its assets, fired a ballistic missile last week toward an airport in Riyadh. The Saudis were able to intercept the projectile, but the event has rattled the region politically.

The UN's humanitarian chief, Mark Lowcock, warned last week that Yemen is facing the world's worst famine in decades in which millions could die, if Saudi Arabia continues to block aid flowing into the country.

Riyadh has prevented aid agencies from landing planes in the country and docking at Yemen's ports, worsening an already dire humanitarian situation, according to Lowcock.

And while Saudi Arabia has said it would take into consideration "the continuation of the entry and exit of humanitarian supplies and crews," Lowcock said that UN planes have not been able to land in the country, and that he believed no other humanitarian agency had been able to gain access.

Yemen is also facing one of the world's worst cholera outbreaks in modern history.

The World Health Organization says there have been more than 900,000 suspected cases of cholera in Yemen since late April, many of them children, and that there are expected to be 1 million cases before the end of the year.

More than 2,000 people have died so far in the cholera outbreak alone.

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Monday's letter to top US officials echoed Lowcock's concerns.

"The recent decision by the Saudi Arabia- and United Arab Emirates-led coalition to temporarily close all of Yemen's land, sea, and air entry points is deeply worrying, coming as it does on top of this already-dire humanitarian situation," the letter said.

"This blockade already has grounded UN flights, prevented humanitarian workers from flying in and out of the country, and barred ships already in port from off-loading lifesaving food, fuel, and medical supplies bound for afflicted civilians, including children."

State Department spokesperson Heather Nauert told reporters last week that the US is closely monitoring the humanitarian crisis in Yemen and is concerned that the recent blockade will only add to the suffering of the Yemeni people.

But while Nauert said that the US supports "unimpeded access for commercial and humanitarian goods" into Yemen," she did not explicitly state support for the UN's call for the Saudis to immediately open borders, airspace and ports used to bring that aid into the country.

The State Department did not immediately respond to CNN's request for comment on Monday's letter.