BANGKOK, April 20 (Reuters) - Thailand will withdraw its 451 medical and engineering troops from Iraq if they are attacked, the Thai prime minister said on Tuesday after a wave of kidnappings and attacks on U.S. allies in Iraq. Thaksin Shinawatra's government is facing growing calls to withdraw the troops, who were sent last year to do humanitarian work in the southern city of Kerbala, 100 km (60 miles) south of Baghdad. "If we get hurt or killed, I will not keep them there," Thaksin told reporters a day after Honduras followed Spain in announcing they would pull their troops...