The US and NATO, hit by an upswing in attacks on Afghan-bound supply convoys in Pak, are stepping up efforts to secure alternative routes amid signs militants have chosen a troubling new tactic in their war with the West. US defense officials said militant attacks on the main overland supply route through Pak, from the seaport of Karachi to the Khyber Pass, had recently grown more frequent and intense, culminating in two daring assaults near the city of Peshawar last weekend. The attacks, which occur beyond the reach of US and NATO forces in Afghanistan, have become more spectacular with the arrival of insurgents driven out of former militant safe havens in the Bajaur region by Pakistan's army and paramilitary Frontier Corps, US military officials said. Navy Adm. Mike Mullen, chairman of the US Joint Chiefs of Staff, acknowledged growing worries about the security of the vital overland route, saying vigorous discussions with Pakistan had been taking place for some time. "We're all increasingly concerned. But in that concern, we've worked pretty hard to develop options," the top US military officer told reporters Wednesday. About 75 percent of the vehicles, parts, weapons, fuel, water and food needed to sustain more than 60,000 Western troops against Afghanistan's Taliban insurgency move through the Khyber Pass and a second overland supply route farther south between Quetta and Kandahar. NATO and the Pentagon have played down recent attacks, calling their effects insignificant. "They were barely measurable on the graph of what goes into Afghanistan on a daily basis," NATO spokesman James Appathurai said. But analysts warn that while the immediate impact has been negligible, convoy attacks constitute a developing threat that could easily become more serious. "If you look at the growth of Taliban influence, they could extend this type of attack over a much broader area," said Anthony Cordesman of the Washington-based Center for Security and International Studies. "And it's not simply our supply route. They're attacking the economic lifeline between Kabul and the outside world." Military planners have examined a number of options, from building up the southern route into Kandahar to flying supplies to countries north of Afghanistan and trucking them south. But US planners have placed an emphasis on finding new routes by sea and land, possibly across the Black Sea and Caspian Sea into Afghanistan's northwest, outside the grasp of Pakistan-based militants. The objective would not be to abandon the overland routes through Pakistan but to make US and NATO forces less dependent on them by adding routes beyond the reach of insurgents. A senior US official said NATO had been talking to Afghanistan's three northern neighbors -- Turkmenistan, Tajikistan and Kazakhstan -- over the past six to 12 months and that fuel was already entering the country from the north. A NATO official said the negotiations had also included Ukraine and would allow the transit of supplies through Russia. Russia has had strained relations with the US and NATO since its August conflict with Georgia. But the NATO official said there had been no interference from Russia. "On the contrary, they also see an interest in having this done as quickly as possible," the official said. In the meantime, US and NATO officials hope for increased security for supply shipments that continue to pass through Khyber. Top military officials in Afghanistan say Pakistan has agreed to double the number of troops assigned to convoy security to two battalions. U.S. and other governments also have urged the commercial contractors who maintain the convoys to add more private security guards. "Companies should also take responsibility," Pakistani Prime Minister Yousaf Raza Gilani said Wednesday.