Security has been stepped up around schools across the country amid calls by sociologists that solutions be found to deep-seated social problems that have led to a spate of attacks around campuses.

?Students at Beijing No 1 Experiment Primary School are escorted by security guards on Thursday.

Premier Wen Jiabao, expressing condolences to the families of the victims, pledged to address the root causes behind the tragedies.

Apart from beefing up security, "we need to handle social problems, resolve disputes and strengthen mediation at the grassroots level", he told Phoenix TV on Thursday.

Minister of Public Security Meng Jianzhu ordered police forces to ensure criminals "dare not and cannot" get their hands on children.

He stressed that security measures in privately-run schools and kindergartens as well as those in remote areas and rural regions should be reassessed to stem risks.

The directive followed Wednesday's deadly attack in Northwest China's Shaanxi province, the fifth on children in the past month.

A 48-year-old local farmer stabbed seven children and two adults to death - the youngest victim aged only 3 - before he killed himself.

Initial police investigations showed that suspect Wu Huanming, who had rented out his house to kindergarten owner Wu Hongying, demanded in April that his house be vacated when the lease expired.

But the victim said she could only do so in June or July, during vacation.

The tragedy shows that privately-run kindergartens as well as those in remote rural areas are a weak link in the tightened security network, experts said.

"In rural areas, especially remote villages, it is impossible for every kindergarten to be guarded by a police officer," said Guo Taisheng, professor and dean of the public security department of the Chinese People's Public Security University.

More importantly, the increased security efforts are only one way of thwarting school assaults, sociologists and experts said.

Law professor Li Yunlong said the five attacks on children in the past month share some common characteristics, such as the attackers were jobless men in their 30s to 40s.

"Severe punishment is not a deterrent because they are not afraid of death, which has been demonstrated in some cases in which the attackers later committed suicide," said Li, who is at the Jiangxi provincial academy of social sciences.

"Their motives are to exact revenge on society and expose social problems, such as unemployment and unfair distribution of wealth," he said.

Fang Changchun, associate professor at the sociology department of Nanjing University, said this group needs a way to vent long-suppressed frustration, which the attackers in the five cases apparently did not find.

"They turned to children to express their resentment because they had no direct targets to do so, and compared to other places, schools and kindergartens are not as heavily guarded," he said.

But before such social problems are resolved, placing schools under police protection is necessary, he said.

Starting Thursday, many cities beefed up security measures.

Police in Changsha of Central China's Hunan province patrolled with sub-machine guns.

In Beijing, 800 well-trained security guards, clad in helmets and armed with tear gas and batons, made their presence felt in primary schools and kindergartens. The municipality aims to equip each school with at least two such security guards.

In Shanghai and Guangzhou, parents need passes to enter school.

Schools also started teaching children on how to respond to abductions or injuries, said Hu Zhijin, deputy principal of Zhenguang Primary School in Guangzhou.

"Though children feel restricted and some are a little afraid, the measures are necessary and reassuring," said an elderly man picking up his granddaughter at Huayang Primary School in Guangzhou.

Despite all the security measures, a woman in her 30s carrying a long knife was caught on Thursday by security guards and traffic police when she ran into a children's activity center in Hangzhou, Zhejiang province, local newspaper City Express reported.

No casualty was reported.