KARIMABAD, Pakistan (Reuters) - Water began seeping out from a lake formed by a landslide in north Pakistan into a spillway Saturday, officials said, who added that the next one or two days are critical to avoid catastrophic flooding.

If the spillway doesn’t contain the water and the landslide dam bursts, authorities fear the heavy flooding could wash away many villages, bridges and roads, affecting up to 50,000 people.

“At this point, the water flow is very smooth but it’s eroding the spillway, widening it,” Gilgit-Baltistan’s Commissioner, Asif Bilal Lohdi, told Reuters by telephone.

“Let’s see how the water behaves in the next four to five hours, then the situation will be clear. We are on high alert,” he said.

Officials are hoping for a gradual erosion of the blockage, but they have not ruled out a major breach due to rising water levels from melting glaciers.

The landslide in early January blocked the Hunza River and created a huge lake near Attaabad village. Twenty people were killed and another 25,000 were left stranded upstream, and now struggle to remain linked to the main town of Gilgit.

The Pakistan military created a spillway to drain the 19 km (12 miles) long and 360-feet deep lake.

Lohdi said a major breach in the dam was possible.

“It could burst and cause flash flooding, which will ultimately wash away 34 villages and a part of the Karakorum Highway (KKH),” he said referring to the main road to China.

Nearly 30,0000 villagers have already been relocated to 24 camps.

Head of the military’s relief organization, Lieutenant General Nadeem Ahmed, said the erosion of the spillway and blockade would be faster in the next one or two days. “These days are very crucial,” he said.

Pakistan is already under fire for its handling of crippling power cuts and is struggling to contain a Taliban insurgency and strengthen its sluggish economy, so it can ill-afford a catastrophe such as widespread flooding.

Before the flow of water began, the lake had already swamped at least four upstream villages, displacing nearly 6,000 people, according to aid workers and residents.

The lake has also submerged a 22-km stretch of the Karakoram Highway, a trade route for a significant portion of Pakistan’s consumer goods from China.

A leading trader in the region said the losses from the disaster already amounted to 1.5 billion rupees ($17.8 million), about a quarter of the annual two-way trade along the highway.

Officials said a detailed survey would be carried out after the water receded to assess the damage to the highway. It would likely take months for the highway to reopen.