The Palestinian Health Ministry on Sunday said it identified the first case of the Omicron variant of coronavirus in the Gaza Strip.
The carrier is a resident of Gaza who was infected within the coastal territory, ministry official Majdi Dhair said.
Mr Dhair said the variant, which was first detected in southern Africa and Hong Kong last month, was now spreading among the Gazan population.
The discovery poses a new challenge to the enclave's under-developed healthcare system.
"We are ahead of difficult days. It is expected that the Omicron variant will spread fast," he said.
Since the beginning of the pandemic, health experts have warned that Covid-19 poses a particular threat to Gaza residents.
With a population of 2.2 million people, living in 362 square kilometres, Gaza is one of the most densely populated areas on Earth.
Since coronavirus first emerged, the coastal enclave has recorded 189,837 Covid-19 infections and 1,691 deaths.
Mr Dhair said about 40 per cent of Gazans had received vaccine shots and urged the rest to get inoculated.
In the Israeli-occupied West Bank, three cases of Omicron variant were detected on December 16 and the number had since risen to 23 among the 3.1 million population, Palestinian health authorities said.
Ministry spokesman Kamal Al Shakrah said the three people live in different cities in the territory and had recently returned from abroad. He did not say which countries they had visited.
In response to the discovery, the ministry said medical teams were tracking down and testing those who had recently come into contact with the three people.
Israel, which captured the West Bank in the 1967 Middle East war, controls movement in and out of the territory.
The Israeli Health Ministry has so far identified at least 67 cases of the omicron variant and has imposed tight travel restrictions, including a ban on most foreign tourists entering the country.
Around 2.5 million Palestinians from both territories have received one dose of a vaccine against Covid-19, while just over 2.1 million Palestinians have received a second dose.