Yemeni Houthi rebels managed to infiltrate Saudi territory
Saudi Arabia has evacuated 240 villages because of fighting with Yemeni rebels, the UN children's fund has reported.
The Unicef statement said more than 50 schools had also been closed since fighting in neighbouring Yemen spilled across the border last week.
The body expressed its "deep concern about the escalation of the conflict".
Northern Yemeni rebels, known as Houthis, infiltrated Saudi territory after a series of clashes with Saudi border police.
A Saudi government official said on Thursday that Saudi forces were using air power and artillery to enforce a 10km-deep buffer zone inside northern Yemen to keep the rebels away from the border.
Riyadh had previously claimed troops were only attacking the rebels inside Saudi territory.
A rebel spokesman said that their strongholds in the Yemeni border region of Saada had been struck by Saudi forces.
"The Saudi artillery continued its bombardment on Friday with shells and rockets landing within the territory of Yemen," a rebel spokesman said.
Saudi troops are enforcing a buffer zone in north Yemen
The Houthi rebels are drawn from the Zaidi Shia community, who are a minority in Yemen but make up the majority in the north of the country.
They first took up arms against the government in 2004, saying they wanted greater autonomy and a greater role for their version of Shia Islam. They complain that their community is discriminated against.
The Yemeni government launched a fresh offensive against the Houthis in August 2009, which has precipitated a new wave of intense fighting.
The Houthis have long accused Saudi Arabia of supporting the Yemeni government in offensives against them, a claim Riyadh and Sanaa had both denied.
The Saudi government acknowledged overtly fighting the rebels when a Saudi border policeman was killed at the beginning of November.
The Houthis deny receiving support from Iran which is a predominantly Shia country. They have also refuted allegations of a sectarian dimension to their struggle.
On Friday a senior Iranian cleric, Ayatollah Ahmad Jannati, accused Saudi Arabia of trying to kill off Shia Islam by attacking the Houthis.
Speaking at Friday prayers in Tehran, the ayatollah also accused Riyadh of closing down Shia mosques in Saudi Arabia.
An austere form of Sunni Islam, known as Wahhabism, is dominant in Saudi Arabia, though there is a small Shia minority.
Riyadh and Tehran have been exchanging warnings about the treatment and conduct of Shia Iranian pilgrims arriving in Saudi Arabia ahead of Hajj ceremonies.
The Hajj is performed by both Sunni and Shia adherents and this year takes place at the end of November.