MONTREAL - Japanese wireless network operator NTT DoCoMo and Research In Motion have suspended sales of the BlackBerry Bold after a small number of devices overheated during charging.

RIM said Friday that the problem appears to be limited to Bold smartphones sold in Japan since last week and the cause hasn't yet been determined.

"Although RIM's analysis of the devices in question has allowed it to rule out a battery problem, the root cause remains under investigation," the Waterloo, Ont.-based company said in a statement.

Temperatures of the Bold smartphones were warmer than usual during charging, but appeared to have remained within regulatory standards, RIM said.

About 30 Japanese users have reported their BlackBerry Bold keyboards heated up during charging, representing less than one per cent of about 4,000 units sold in Japan since the Bold's recent launch.

Various models of the BlackBerry smartphones are available in more than 150 countries from over 425 carriers. NTT DoCoMo has more than 54 million customers in Japan.

Shares of Research In Motion were trading late Friday morning at $50.98, up $1.16 or 2.3 per cent, on the Toronto Stock Exchange.