U.S. sales of medium- and heavy-duty trucks in June kept the string of double-digit increases intact in the year’s first half with a 10.3% gain on like-2009.

The last year-over-year decline was 13.8% in December.

Class 6 accounted for took the lion’s share of the increase, with June sales surging 77.1% to 3,070 units from 1,733 year-ago.

Related document: U.S. Truck Sales by Company by GVW - June 2010

International Truck & Engine Corp. provided the biggest push, with deliveries last month climbing 75.7% to 1,536 from 874 in like-2009. Daimler AG’s Freightliner brand sales tripled to 643 units from 212, sending the truck maker’s total deliveries in Class 6 up 174.2% to 743 from 271.

Class 5 June sales rose 12.7% to 2,613 units from 2,318 year-ago. Ford Motor Co. witnessed the biggest increase with 1,547 deliveries, up 47.6% from year-ago’s 1,048.

Class 4 sales slipped 24.4% to 1,364 from 1,805 in like-2009. Ford posted the greatest decline, slipping 79.3% to 194 from 939.

Class 7 saw a 7.8% drop to 3,309 from 3,587 units in June, as nearly every truck maker in the segment suffered losses. International led the decline, with sales down 10.5% to 1,185 from 1,324 in like-2009. Daimler’s 16.8% gain to 1,311 from 1,122 deliveries could not offset across-the-board weakness.

Medium-duty sales in June rose 9.7% to 10,356 units from 9,443 year-ago. Through the year’s first half, medium-duty deliveries were up 6.6% to 54,877 from 51,464, compared with prior-year.

Class 8 June sales rose 11.1% to 8,896 units from 8,006 in like-2009.

Daimler led the heavy-duty segment, with deliveries climbing 29% to 2,920 units, from 2,264 year-ago. Its Freightliner brand was the best-selling nameplate in the class, posting an increase of 37.4% to 2,658 from 1,934.

International sold 2,884 units in Class 8 for June, up 4.8% from prior-year’s 2,752.

In other big-truck news, Portland, OR-based Daimler Trucks North America will call back workers to ratchet up production this month, building 67% more Class 8 heavy-duty units in July than in like-2009.

The truck maker says so far this year it has received 16,565 orders for Freightliner-brand trucks. Including school buses, DTNA’ order bank now totals 21,400 units.

The production uptick will lead to the call-back of 540 workers at North Carolina assembly plants in Mt. Holly, and a Gastonia components-and-logistics facility in the state.

DTNA says given its 34.2% share of the Class-8 segment in the U.S. and Canada, its order book now accounts for more than one-third of the industry’s backlog.