Three Turkish civil engineers, at least two Afghan police officers and one foreign soldier were killed Monday in separate attacks in Afghanistan as Taliban insurgents issued a warning against participation in the second round of presidential voting, slated for June 14.

The engineers were traveling in a car when a suicide bomber rammed into it with a motorcycle rigged with explosives in eastern Nangahar province, near the border with Pakistan, the Interior Ministry said. It said one other Turkish engineer and an Afghan were injured.

The Turks worked for a construction firm and were building a base for the Afghan police.

A spokesman for the Taliban, Zabiullah Mujahid, said he could not immediately assert or deny responsibility for the attack, which coincided with renewed violence since the radical Islamist movement announced the launch of its spring offensive last month.

The foreign soldier, a member of the U.S.-led international coalition, which includes NATO forces, was killed in an insurgent attack in eastern Afghanistan, the coalition said. It did not identify the service member or provide details of the attack or its exact location.

The two Afghan police officers died during a clash with insurgents who stormed a district headquarters in southern Helmand province, officials said.

Another Taliban spokesman, Qari Yousef Ahmadi, said the assailants included suicide bombers. He asserted that “dozens” of police officers were killed during the fighting, which he said lasted several hours.

The attacks came less than two weeks before a runoff election to choose a successor to President Hamid Karzai, who was barred from running for reelection because of term limits. The Taliban staged scores of relatively small-scale attacks during the first round of voting on April 5, but voters in many parts of the country were not deterred from participating in what has been described as the first peaceful transfer of power in Afghan history.

Ahmadi, the Taliban spokesman, vowed Monday that the insurgents would sabotage the runoff and warned people against voting in it.

“We would urge people to refrain from taking part in this sham election,” he said. “We will target polling centers and election offices with whatever means we have.”

The runoff pits former foreign minister Abdullah Abdullah, who won a plurality of the vote in April, against Ashraf Ghani, a former finance minister who finished second.