Obama made the announcement on Monday after being briefed by Dr Thomas Frieden, the director of the Centers for Disease Control and Prevention (CDC).

However, Obama did not specify how screening would be changed.

Frieden said that officials would explore a variety of options.

Obama also called for more help from other countries in fighting the outbreak in Africa.

In Dallas, where the Ebola patient Thomas Eric Duncan remains in critical condition despite taking a trial drug, officials acknowledged that their city was on edge as it waited to learn if those he stayed with and medical workers became infected while caring for him.

According to CDC, the maximum incubation period for the virus is 21 days, but symptoms typically appear in eight to 10 days.

Duncan, a 42-year-old Liberian who apparently contracted the virus before flying to the US, began complaining of symptoms on September 24 and was hospitalised in isolation on September 28.

The 10-day threshold, therefore, will pass by the middle of this week.

Thus far, none of the 10 people who officials say are at high risk — three who shared an apartment with Duncan and seven health care workers — have displayed any symptoms.

An additional 38 people who are considered at lower risk are also being monitored by workers who check their temperature twice daily.

Duncan remained in critical condition on Tuesday at Texas Health Presbyterian Hospital.

The hospital announced that doctors began administering doses of an experimental drug called brincidofovir on Saturday.

The drug, which is being developed by the biotechnology company Chimerix, is being tested against various viruses in clinical trials.

Chimerix announced on Monday that the Food and Drug Administration has agreed to make the drug available to treat Ebola on an emergency basis.

It had never been tried before in people infected with Ebola, and there is no data showing it works even in animals.

But test-tube experiments in government labs suggest the drug might be effective, the company added.

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