Nepal has urged airlines flying into the country's only international airport to use smaller planes after holes and cracks were discovered on the runway, a top aviation official said Thursday.

Airport officials have written to all 26 international airlines asking them not to use wide-bodied planes for their flights to Kathmandu amid safety concerns over the runway, the head of the Civil Aviation Authority of Nepal (CAAN) said.

"We have requested them not to use big aircraft because with their huge loads, they tend to put pressure on the airport and cause damage to the runway," Ratish Chandra Lal Suman, the authority's director general, told AFP.

He said the surface of sections of the 3,000-metre (10,000-foot) long runway has lifted causing small holes, while cracks have also been found.

Some flights were delayed at Tribhuvan International Airport and others were diverted to Lucknow in neighbouring India as well as the Bangladesh capital, while ground crews conducted repairs.

"Most flights are operating during the day, so we can repair only for about 20 minutes. During the night, we spend more time on the repair," he said.

Although all airlines have been notified only some -- including Air Asia, Etihad Airways, Qatar Airways, Thai Airways and Korean Air -- operate the wide-body Boeing 777s and Airbus 330s through the airport, he said.

Suman said the notification was "a request and not a ban," adding that "it would be better if they can fly smaller aircraft."

He said it was unclear what caused the cracks and the government has commissioned a Spanish contractor to submit a report on the runway conditions which was expected by September.

"We are waiting for the report. When it's presented, we will know for sure what is causing the damage to the runway," he said.

In June 2011, fights were disrupted at the airport for several hours when a section of the runway subsided, apparently due to strong monsoon rains.

The government pledged then to investigate, promising to form a high-level committee but little resulted from it.

Experts at the time pointed to weak subsoil under the runway, which was threatened by rivers and waterways that flow around the 50-year-old airport.

Suman said he was aware of the possible impact on tourism, which is a mainstay of the tiny Himalayan nation.

"We know that it will impact the tourism sector. But we also have to save the airport from further damage," Suman said.

An average of 70 planes land and depart from Kathmandu on a daily basis.

Every day, around 4,000 people arrive by air into Kathmandu, while the number of foreign visitors peaks during September and October, according to Nepal's tourism ministry.