Targeted killings of terrorism suspects have increased under US President Obama\'s administration. Germany, however, has so far avoided commenting on the practice that some have called a violation of international law. Exactly who were the nine people killed when a US drone fired rockets at a house in the Pakistani province North Waziristan on May 5 is unknown. That\'s the case for the attack that took place on May 5, as well as for nearly all drone attacks and targeted killings conducted by US spies in the CIA. Since President Barack Obama took office, as many as 2,355 people have been killed by drone strikes in Pakistan - in the final five years of the former President George W. Bush that number was only 426. The government in Pakistan has reacted with increasing anger. At the end of April, the Pakistani Parliament unanimously voted to call for an end to US attacks in Pakistan. \"Contravention of international law\" \"These drone attacks are illegal,\" said a spokesman for the Foreign Ministry in Islamabad. \"It is a violation of Pakistan\'s sovereignty and territorial integrity and are in contravention of international law.\" People marching in protest against US drone attacks People in Pakistan took to the streets to protest US drone strikes Obama\'s strategy also puts German politicians in a difficult spot, according to Peter Rudolf, head of the Americas research division at the German Institute for International and Security Policy, because the United States could use information provided by German intelligence agencies in the attacks. Rudolf also said German citizens have been killed by US drone attacks. In October 2010, 20-year-old German Bünyamin E. was killed a few weeks after he and his brother traveled to Pakistan looking for training to become terrorists. At the beginning of March another German, identified as Samir H., was killed along with several other people thought to be members of the Taliban, according to a report in Germany\'s Der Spiegel news magazine. The German government has been restrained in its comments on both incidents. \"The German embassy in Islamabad and the foreign ministry are working for clarification,\" said a spokesman for the foreign ministry in Berlin. Germany has not officially confirmed either of the deaths. \"A touchy topic\" \"It is, of course, a touchy topic when a German citizen, who from the American point of view is engaged in terrorist activities, is killed abroad,\" Rudolf said. \"Should a case be brought against the intelligence services of an allied country?\" A pilot in front of computer screens controls a drone Pilots in the United States control military drones That has not happened, but prosecutors have been examining the case of Bünyamin E. for the past year and must determine whether it has the jurisdiction to file suit. \"The question especially is whether the alleged incident occurred as part of an armed conflict as defined by international law,\" a spokesman said. \"That is the only way in which federal prosecutors would have jurisdiction to further examine the alleged incident.\" Whether drones are used as part of a war is also a key issue when it comes to international law, as only then could such attacks be deemed legal. But even then, the legality of drone strikes would depend on the circumstances, according to Rüdiger Wolfrum, director of the Max Planck Institute for Comparative Public Law and International Law. If a Taliban commander were to be killed in Pakistan without loss of other life, the attack would likely be legal, he said. \"The justification for encroaching on the territorial integrity of Pakistan would be that Pakistan is not willing or able to control its own territory in a manner to ensure it is not used for attacks across its border,\" Wolfrum said. However, he also added that if a disproportionate number of civilians were also killed in the attack it would represent a war crime. Solid legal ground Matthew Waxman, a professor at Columbia Law School in New York and member of the Council on Foreign Relations, used a similar logic to explain the legality of the US policy of drone warfare. Hakimullah Mehsud Hakimullah Mehsud, left, died in a drone attack in 2009 \"In general, lethal force is legally permissible against enemy fighters in an ongoing war, and such force may be used on the territory of a foreign state, if that state consents, or if it is unwilling, or unable, to take action itself to deal with the threat,\" said Waxman, who served in the Bush administration. \"The United States is on strong legal footing in its actions against al Qaeda fighters and their allies across the Pakistani border. It is difficult to assess the legality of any specific drone strike in Pakistan without much more information than is publicly available,\" he says. The public, however, knows very little about the circumstances of the targeted killings. It was only a few months ago that officials in the Obama administration roughly outlined the criteria used to decide on targets. \"We consider the advantages and disadvantages of taking action,\" John Brennan, an assistant to Obama for homeland security and counterterrorism, said in a speech in Washington on April 30. \"We also carefully consider the costs of inaction and whether a decision not to carry out a strike could allow a terrorist attack to proceed and potentially kill scores of innocents.\" The US government has relied on the right to self-defense granted by the UN Charter, which it argues covers attacks outside areas of immediate conflict in Afghanistan and includes countries like Somalia and Yemen. Germany skirts the issue The German government has avoided commenting on US drone attacks. A Predator drone flies over Kandahar Air Field, southern Afghanistan Where and how drone attacks occur can determine their legality \"International law calls for more detailed information about the individual case,\" said a spokesman for the foreign ministry, adding that the government did not have access to that information. It seems, however, that Berlin has proven unwilling to support the practice, according to Rudolf. \"Certain information that would lead to the killing of a German citizen is not permitted to be shared as part of the cooperation between intelligence agencies,\" he said. According to media reports, information provided to the United States contains provisions that it only be used to take suspects into custody. A spokesman for the interior ministry, which oversees intelligence gathering, did not confirm this limitation. \"Our constitutional order prohibits providing information regarding extra-judicial killing,\" the spokesman said.