The number of deaths from traffic accidents in the United States jumped 8.1 percent in the first half of 2015, suggesting smartphones and other driving distractions could be making America's roadways more dangerous, officials said on Tuesday.

Preliminary government statistics, released during a Thanksgiving holiday week known for heavy traffic congestion, showed deaths rising to 16,225 in the January-June period at a rate more than double an increase in overall driving spawned by falling gasoline prices and a growing economy.

"The increase in smartphones in our hands is so significant, there's no question that has to play some role. But we don't have enough information yet to determine how big a role," said Mark Rosekind, who heads the National Highway Traffic Safety Administration, the federal government's auto safety watchdog.

The jump in 2015 fatalities follows a decline in annual traffic deaths to 32,675 last year, for a record low of 1.07 deaths per million vehicle miles traveled, according to NHTSA statistics. The 2014 data included 21,022 passenger vehicle deaths, the lowest since record-keeping began in 1975.

The increase in the first half of 2015 was the biggest six-month jump in traffic deaths reported since 1977, according to statistics. But officials cautioned that semi-annual results can be subject to major revisions and noted that a comparable 7.9 percent increase in early 2012 led to a 4 percent rise for that year as a whole.

Officials said it was too early to identify contributing factors. But Rosekind told reporters that officials are looking at likely causes including distracted driving and the possibility lower gas prices have encouraged more driving among "risky drivers" such as teenagers.

Rosekind also criticized an absence of effective state laws that prohibit hand-held smartphones by drivers or require the use of seatbelts and motorcycle helmets.

The auto safety agency expects to unveil a program next year to target $500 million in federal safety grants at human factors that are responsible for 94 percent of motor vehicle crashes.