Saudi Arabia is building a giant border fence to seal off its troubled frontier with Yemen, Saudi border authorities have told the BBC.
The 1,800km (1,100-mile) fence is set to run from the Red Sea coast in the west to the edge of Oman in the east.
Security has deteriorated on the Yemeni side after its long-term president stepped down in 2012, Saudi border official Lt Col Hamed al-Ahmari said.
Yemen is considered a base of al-Qaeda in the Arabian peninsula (AQAP).
President Ali Abdullah Saleh was forced to hand over power after 33 years to his deputy, Abdrabbuh Mansour Hadi, in February last year after months of anti-government protests. Since then, militants have gained ground in the country.
Border security has dramatically worsened in the aftermath of the revolution, as thousands of illegal immigrants, drug smugglers and gun runners try to slip from impoverished Yemen into Saudi Arabia, one of the world's richest countries, Lt al-Ahmari told the BBC's Frank Gardner.
Five Saudi border guards had recently been killed along the border in shoot-outs with well-armed smugglers, he added.
The first part of the fence has already been built on the coast, slowing down - but not stopping - the tide of illegal immigrants.
Smugglers and militants have already shown ingenuity to circumvent the new border by using women, mules and secret paths to get into Saudi Arabia, our correspondent reports.
Earlier this month, UN-backed reconciliation talks began in Yemen aimed at drafting a new constitution and preparing for full democratic elections next February.