The following nationalities can enter without a visa;
Azerbaijan, Armenia, Belarus, Cuba, Georgia, Japan, Kazakhstan, North Korea, Malaysia, Moldova, Mongolia, Russia, Tajikistan, Turkey, Ukraine Uzbekistan and Vietnam.
Visitors from Australia, Austria, Bahrain, Belgium, Bosnia and Herzegovina, Brunei, Canada, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Kuwait, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, Netherlands, New Zealand, Norway, Poland, Portugal, Qatar, Republic of Korea, Saudi Arabia, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Arab Emirates, United Kingdom, United States of America and Vatican, do not need visas for entry up to 60 days.
In most cases you do not require an invitation for a visa. Visas cost US$110 and up.