They said they were under instructions not to allow them to stay.
"We have returned around 400 refugees back into Somalia. We put them on lorries and sent them back home," a police commander told AFP.
In addition, police said they intercepted and sent home about 40 other refugees who were trying to enter the country through the northern Ijara district.
The refugees had been fleeing fighting in Somalia, where government troops backed by Ethiopian forces are in the pursuit of vanquished Islamic Courts fighters.
Kenya already hosts at least 160,000 refugees who fled fighting in more than 15 years of unrest in Somalia.
Somalis stranded
The UN Office for the Co-ordination of Humanitarian Affairs said on Tuesday that about 4,000 Somalis were stranded in the Somali town of Dhobley waiting to cross into Kenya.
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"All the ICU did was bring peace to a region engulfed in calamity for more then a decade"
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The Kenyan government also blocked the Kenya Red Cross Society from entering the refugee reception centre in Liboi, about 550km northeast of the capital, where about 360 refugees were staying, UN officials said.
"We are certainly disturbed by reports we are getting that people seeking asylum are being sent back because this amounts to contravention of the international humanitarian law," said Millicent Mutuli, the spokeswoman for UN refugee agency (UNHCR).
For his part, Raphael Tuju, Kenya's foreign affairs minister, said: "There's no reason at all to allow an influx of people unless there are women and children and it's really, really obvious that they are in danger in their own country. At this particular time, we don't see that danger."
US forces patrolling
The US forces, based in Djibouti, are patrolling the seas off Somalia in a bid to capture leaders of the Islamic Courts militia, including al-Qaeda agents wanted for the 1998 bombings of US embassies in East Africa, Sean McCormack, the state department spokesman, said on Thursday.
He said: "We would be concerned that no leaders or members of the Islamic Courts, which have ties to terrorist organisations including al-Qaeda, are allowed to flee and to leave Somalia."
McCormack said Washington was working closely with Somalia's Horn of Africa neighbours "to ensure that these individuals aren't able to transit those borders and exit Somalia.
"The other countries in the region don't want to see that any more than we do".