BOSTON -- Strong demand for air freight in key markets and freight backlogs are delaying shipments and pushing rates higher.

In certain gateways, rate increases have hit 30%, Trans Global Logistics said in an email to customers.

The demand for air lift is in line with seasonal patterns, the firm said, although the surge for capacity hit "about a week earlier than normal."

March often ends the first fiscal quarter, the firm noted, which in turn ignites a surge in electronics exports. "High-tech importers continue to tell our operations staff that demand should be strong later this month."

Sites with major backlogs include Bangladesh, Seoul, Taiwan, Sri Lanka and Vietnam.

Meanwhile, Hong Kong, Shanghai and Taiwan have seen prices increase as much as 30%. In other areas, carriers will only accept cargo at express rates, which can add another $1.50/kg. to the cost of shipping by air.

Some carriers have announced new fuel surcharge increases as well.