The battle for Hodeidah is expected to be by far the biggest of a three-year war between an alliance of Arab states and the Iranian-backed Houthis.
The port city of Hudaida is crucial for incoming aid, food and medicine for a nation driven to the brink of starvation by the conflict and a Saudi-led blockade.
People fled the Red Sea port city on Wednesday as the Houthis deployed military vehicles and troops in the city centre and near the port, while Arab warplanes flew overhead.
The news came Wednesday night after United States military officials said that the U.S. is helping the Saudi-led coalition choose targets that will minimize civilian casualties. In August 2015, air strikes disabled four giant mobile cranes, drastically slowing the unloading of food until they were replaced by the USA - which supports the coalition - this January. "We thought it could not get any worse, but unfortunately we were wrong".
The initial battle plan appeared to involve a pincer movement.
Lowcock briefed the U.N. Security Council in a closed-door session on Monday and spoke to reporters afterwards.
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Boys hold posters with crossed-out pictures of President Trump during an April march against plans by the Arab coalition to attack Hodeidah, Yemen.
Hodeida is controlled by Houthi rebels who are fighting the Saudi-led coalition, and the coalition believes the port has been key to the rebels smuggling in arms. Houthi forces have fired missiles at ships previously.
The Saudi-led coalition did not immediately acknowledge the incident.
The government forces of Yemeni President Abd Rabbuh Mansur Hadi fight the predominantly Shia-led Houthis with the help of Saudi Arabia and its allies.
Hudaydah, 140km (90 miles) west of the capital Sanaa, was Yemen's fourth-largest city and a major economic hub before rebels took control of it in late 2014.
The United Nations and other aid groups had pulled their global staff from Hodeida ahead of the rumored assault.
And just to the north is the Ras Isa oil terminal - which served the Marib oilfields and was the country's main export terminal - and the nearby port of Saleef.
United Nations pulls staff from Yemen’s Hodeidah ahead of expected imminent assault
Mr Griffiths held several rounds of talks last week in the Yemeni capital of Sanaa, which is held by the rebels. Meanwhile, the U.S. was following developments in Yemen very closely, said Secretary of State Mike Pompeo.
Sulaiman Almazroui, the UAE's ambassador to Britain, rejected warnings that United Nations peace efforts were now caught up in the fate of Hodeidah.
The organisation also expressed concern about Hudaydah's water and electricity networks, which it said were vital to the population's survival.
David Miliband, the head of the International Rescue Committee, called the offensive "an attack on the political and diplomatic process to bring peace to Yemen".
"If the current situation continues we will have a stalemate politically", he said. The Saudi-led coalition has been criticized for its airstrikes killing civilians.
Other aid officials warn an attack on the port would set back chances of a political settlement and make it hard for the flow of aid to continue.
The UN says some 600,000 people live in and around Hudaida, and "as many as 250,000 people may lose everything - even their lives" in the assault.
United Nations refugee chief Filippo Grandi said there was a danger Yemenis might try to flee across the sea to Somalia or Djibouti. The sound of heavy, sustained gunfire clearly could be heard. "This is possibly what we're most concerned about". "Some workers have left to their villages for fear of the war", said Mohamed, a Hodeida resident who gave only his first name for fear of reprisals.
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