* Security forces ordered to impose new strict measures in Liberia
* Isolation unit in capital overflowing, new ward delayed by protests
* Liberia faces "humanitarian crisis" - assistant health minister
* U.S. charity fighting virus withdraws non-essential staff (Recasts with Liberia's national plan)
By David Lewis and Emma Farge
DAKAR, July 30 (Reuters) - Liberia announced on Wednesday the quarantine of a number of communities and the closure of schools across the country, the toughest measures yet imposed by a West African government in a bid to halt the worst Ebola outbreak on record.
Security forces in Liberia were ordered to enforce the measures, part of an action plan that included placing all non-essential government workers on 30-day compulsory leave.
As of July 23, 672 deaths have been blamed on Ebola in Liberia, neighbouring Guinea and Sierra Leone, according to World Health Organisation figures. Liberia accounted for just under one-fifth of those deaths.
"This is a major public health emergency. It is fierce, deadly and many of our countrymen are dying and we need to act to stop the spread," Lewis Brown, Liberia's information minister, told Reuters.
"We need the support of the international community now more than ever. We desperately need all the help we can get."
Referring to the orders issued to the security forces to impose the plan, he added: "We are hoping there will be a level of understanding and that there will not be a need for exceptional force."
Earlier on Wednesday, Liberian health officials said an isolation unit for Ebola victims in Liberia's capital, Monrovia, was overrun with cases and health workers are being forced to treat up to 20 new patients in their homes.
Protests by the local community against the construction of a new isolation unit at Elwa Hospital have ended, said Tolbert Nyenswah, an assistant minister of health, but patients with Ebola symptoms will have to wait at home until work is finished.
"The staff here are overwhelmed. This is a humanitarian crisis in Liberia," Nyenswah told Reuters by telephone. "People are being given care at their homes until we can move them to the new unit."
Nyenswah said the suspected patients were being treated by trained medical staff with full protective gear, but it would take at least 24-36 hours to build the new unit.
Initial resistance to building a new isolation unit highlighted the fear and mistrust health workers have faced across West Africa as they battle the outbreak, which has strained the region's weak health systems.
Dozens of local health workers have died treating patients and two Americans working for Samaritan's Purse, a U.S. charity operating in Liberia, were infected over the past week.
Samaritans Purse said on Wednesday that Kent Brantly, a doctor working for Samaritan's Purse, and Nancy Writebol, a colleague who was also volunteering in Liberia, had shown a slight improvement but were still in serious condition.
The organisation said that it was withdrawing non-essential staff from the country because of instability and security issues. (Writing by David Lewis; Editing by Daniel Flynn)