Afghan police officers secure a transportation headquarters that was besieged for two days last weekend, on May 14, 2011 in Kandahar, Afghanistan. Taliban gunmen and suicide bombers unleashed a major assault May 7, 2011 on locations in Kandahar that included the governor's compound, the mayor's office, police stations, and an intelligence agency headquarters. (Photo by Majid Saeedi/Getty Images)

Lady officers of the Philippine National Police force show the taped muzzle of their guns at a police camp in Taguig City, south of Manila, Philippines, 22 December 2014. Law enforcement units all over the country held a ceremonial taping of firearms as a counter measure against cases of indiscriminate firing and illegal discharge of service weapons during the upcoming Christmas and New Year celebrations. (Photo by Francis R. Malasig/EPA)

A security exercise takes place along the River Thames on January 19, 2012 in London, England. The exercise including around 44 police officers, 94 military personnel, 15 boats and a Royal Navy Lynx helicopter was conducted by both the Metropolitan Police and the Royal Marines and designed to test their joint capability ahead of the 2012 London Olympic Games. (Photo by Dan Kitwood/Getty Images)

Employees work at a sorting centre of Zhongtong (ZTO) Express ahead of the Singles Day shopping festival, Chaoyang District, Beijing, November 8, 2015. On China's giant Singles Day internet shopping festival, the country's delivery firms are stretched so thin that they are looking for tie-ups, listings and new investors to husband their resources. (Photo by Jason Lee/Reuters)

Tourists standing near the sea shore are hit by a wave, which surged past a barrier under the influence of Typhoon Chan-hom, in Qingdao, Shandong province, China, July 12, 2015. One of the most powerful typhoons to strike eastern China in decades disrupted air, rail and sea transport on Saturday after forcing the evacuation of more than a million people from the provinces of Zhejiang and Jiangsu, state media reported. (Photo by Reuters/Stringer)

A man carries a bag on his back in Guangzhou, Guangdong province, January 26, 2016. According to traffic police, over 2.9 billion trips will be made around China during the 40-day Spring Festival travel rush, which started on January 24. (Photo by Reuters/Stringer)

A bus goes through a flooded area in Jiujiang, Jiangxi Province, China, June 19, 2016. A week of torrential rain in southern China has killed dozens of people and forced relocation of tens of thousands of residents, media reported. (Photo by Reuters/China Daily)

Local residents crowd a swimming pool during a hot weather in Suining, Sichuan province, 27 July 2008. (Photo by Reuters/Stringer)