By Patrick Quinn, Associated Press–

Mon May 16, 3:34 pm ET

KABUL, Afghanistan –

Four American soldiers serving with NATO forces in Afghanistan died Monday in an explosion in the country's south, NATO and a defense department official said, bringing home the human cost of the U.S.-led push into Taliban strongholds.

The official said they were hit by an improvised explosive device. He spoke on condition of anonymity because relatives of those killed were still being notified. The latest deaths make a total of 16 NATO service members killed so far this month, and 167 so far this year.

Afghan President Hamid Karzai, who has often called on Pakistan for help with the Taliban, has been pushing for reconciliation with the insurgents. The United States has also been promoting what it has called a diplomatic surge to help find a nonmilitary solution to the fighting.

Rodriguez predicted that violence would increase further this summer as the Taliban try to retake territory they lost in southern Kandahar and southwestern Helmand provinces in the past year.

"We have said for several months that we will see a rise in violence this spring and summer. And as the Afghan government keeps gaining support from the people, the insurgents will continue to launch sensational attacks against soft targets," Rodriguez said.

He added that Afghan and coalition forces would try to expand security around Kandahar and Helmand and link the two regions.

"These were the two centers of the Taliban movement and the combined team holds them and intends to retain them. We know that the Taliban want them back and we expect, and we are seeing him, continue to attack. But everyday we hold them is one day more to build and harden the environment," said Australian Maj. Gen. Michael Krause, the coalition's deputy chief of staff for plans.

The Taliban recently launched a long-promised spring offensive and have carried out attacks in the southern and eastern parts of Afghanistan, near the border with Pakistan. The Taliban and other insurgent groups retain bases in safe havens across the border in Pakistan's lawless tribal areas.

A Taliban mortar attack Monday in eastern Kunar province left three Afghan civilians dead and eight wounded. The intended target was a U.S. base, Combat Outpost Honaker Miracle, but the four mortars missed it, NATO said.

"Our Afghan National Security and coalition forces are working diligently to counter the recently announced Taliban spring offensive," said Afghan National Army Maj. Shereen Agha, 2nd Brigade, 201st Corps.

The coalition's director of public affairs, U.S. Rear Adm. Vic Beck, said "each of these heinous attacks turns support away from their cause and strengthens the resolve of the people against them."

NATO and the United States hope to slowly start relinquishing control for security to Afghan forces, with the eventual goal of handing over responsibility at the end of 2014. President Barack Obama has said the United States, with about 100,000 troops on the ground, will begin a gradual drawdown in July — with the number to be determined by the situation at the time.